Semiconductor device and driving method thereof

ABSTRACT

A semiconductor device including a memory cell formed using a wide bandgap semiconductor, for example, an oxide semiconductor is provided. The semiconductor device includes a potential change circuit having a function of outputting a potential lower than a reference potential for reading data from the memory cell. With the use of the wide bandgap semiconductor, an off-state current of a transistor included in the memory cell can be sufficiently reduced, and the semiconductor device which can hold data for a long period can be provided.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a semiconductor device using asemiconductor element and a method for driving the semiconductor device.

2. Description of the Related Art

Storage devices using semiconductor elements are broadly classified intotwo categories: a volatile device that loses stored data when powersupply stops, and a non-volatile device that holds stored data even whenpower is not supplied.

A typical example of a volatile storage device is a dynamic randomaccess memory (DRAM). A DRAM stores data in such a manner that atransistor included in a storage element is selected and charge isaccumulated in a capacitor.

When data is read from a DRAM, charge in a capacitor is lost on theabove-described principle; thus, another writing operation is necessarywhenever data is read. A data holding period is short because chargeflows from/into a transistor forming a memory element by leakage currentbetween a source and a drain in an off state (off-state current) or thelike even when the transistor is not selected. For that reason, anotherwriting operation (refresh operation) is necessary at predeterminedintervals, and it is difficult to sufficiently reduce power consumption.Further, since stored data is lost when power supply stops, anadditional storage device using a magnetic material or an opticalmaterial is needed in order to hold the data for a long time.

Another example of a volatile storage device is a static random accessmemory (SRAM). An SRAM holds stored data by using a circuit such as aflip-flop and thus does not need refresh operation, which is anadvantage over a DRAM. However, cost per storage capacity is increasedbecause a circuit such as a flip-flop is used. Moreover, as in a DRAM,stored data in an SRAM is lost when power supply stops.

A typical example of a non-volatile storage device is a flash memory. Aflash memory includes a floating gate between a gate electrode and achannel formation region in a transistor and stores data by holdingcharge in the floating gate. Therefore, a flash memory has advantages inthat the data holding time is extremely long (almost permanent) andrefresh operation which is necessary in a volatile storage device is notneeded (e.g., see Patent Document 1).

However, a gate insulating layer included in a storage elementdeteriorates by tunneling current generated in writing, so that thestorage element stops its function after a predetermined number ofwriting operations. In order to reduce adverse effects of this problem,a method in which the number of writing operations for storage elementsis equalized is employed, for example; however, a complicated peripheralcircuit is needed to realize this method. Even when such a method isemployed, the fundamental problem of lifetime is not solved. In otherwords, a flash memory is not suitable for applications in which data isfrequently rewritten.

In addition, high voltage is necessary for injecting charge in thefloating gate or removing the charge, and a circuit for generating highvoltage is also necessary. Further, it takes a relatively long time toinject or remove charge, and it is not easy to perform writing anderasing at higher speed.

REFERENCE

[Patent Document]

Patent Document 1: Japanese Published Patent Application No. S57-105889

SUMMARY OF THE INVENTION

In view of the above problems, an object of one embodiment of thepresent invention is to provide a semiconductor device with a novelstructure, in which stored data can hold even when power is not suppliedand there is no limitation on the number of write cycles.

In one embodiment of the present invention, a semiconductor device isformed using a material which can sufficiently reduce off-state currentof a transistor, e.g., an oxide semiconductor material which is a widebandgap semiconductor. When a semiconductor material which allows asufficient reduction in off-state current of a transistor is used, thesemiconductor device can hold data for a long period.

For example, one embodiment of the present invention is a semiconductordevice including a memory cell formed using a wide bandgapsemiconductor. The semiconductor device includes a potential changecircuit having a function of outputting a potential lower than areference potential for reading data from the memory cell.

Specifically, structures described below can be employed, for example.

One embodiment of the present invention is a semiconductor deviceincluding a memory cell array including m×n memory cells, a first drivercircuit including a reading circuit, and a second driver circuit. One ofthe memory cells includes a first transistor including a first gateelectrode, a first source electrode, a first drain electrode, and afirst channel formation region; and a second transistor including asecond gate electrode, a second source electrode, a second drainelectrode, and a second channel formation region. The first channelformation region includes a different semiconductor material from thesecond channel formation region. The reading circuit includes a load; aclocked inverter; and a third transistor including a third gateelectrode, a third source electrode, a third drain electrode, and athird channel formation region. An output terminal of the clockedinverter is connected to the third source electrode or the third drainelectrode of the third transistor.

In the above structure, the first source electrode is connected to asource wiring, an input terminal of the clocked inverter is connected tothe first drain electrode and the second drain electrode via a bit line,and the second gate electrode is connected to the first gate electrodeand the second source electrode via a gate line.

One embodiment of the present invention is a semiconductor deviceincluding a memory cell array including m×n memory cells, a first drivercircuit including a reading circuit, and a second driver circuit. One ofthe memory cells includes a first transistor including a first gateelectrode, a first source electrode, a first drain electrode, and afirst channel formation region; a second transistor including a secondgate electrode, a second source electrode, a second drain electrode, anda second channel formation region; and a capacitor. The first channelformation region includes a semiconductor material different from asemiconductor material of the second channel formation region. Thereading circuit includes a load; a clocked inverter; and a thirdtransistor including a third gate electrode, a third source electrode, athird drain electrode, and a third channel formation region. An outputterminal of the clocked inverter is connected to the third sourceelectrode or the third drain electrode of the third transistor.

In the above structure, the first source electrode is connected to asource line, an input terminal of the clocked inverter is connected tothe first drain electrode and the second drain electrode via a bit line,the second gate electrode is connected to a gate line, one electrode ofthe capacitor is connected to a capacitor line, and the other electrodeof the capacitor is connected to the first gate electrode and the secondsource electrode.

In the above semiconductor device, the first transistor is a p-channeltransistor and the second transistor is an n-channel transistor.

In the above semiconductor device, the second channel formation regionof the second transistor includes an oxide semiconductor.

Note that the above described transistor includes an oxide semiconductorin some cases; however, the present invention is not limited to this. Amaterial which can realize the off-state current characteristicsequivalent to those of the oxide semiconductor, such as a wide gapmaterial like silicon carbide (specifically, a semiconductor materialwhose energy gap Eg is larger than 3 eV) may be used.

Note that in this specification and the like, the term such as “over” or“below” does not necessarily mean that a component is placed “directlyon” or “directly under” another component. For example, the expression“a gate electrode over a gate insulating layer” can mean the case wherethere is an additional component between the gate insulating layer andthe gate electrode. The terms such as “over” and “below” are simply usedfor convenience of explanation.

In addition, in this specification and the like, the term such as“electrode” or “wiring” does not limit a function of a component. Forexample, an “electrode” is sometimes used as part of a “wiring”, andvice versa. Furthermore, the term “electrode” or “wiring” can includethe case where a plurality of “electrodes” or “wirings” is formed in anintegrated manner.

Functions of a “source” and a “drain” may be replaced with each otherwhen a transistor of opposite polarity is used or when the direction ofcurrent flowing is changed in circuit operation in some cases.Therefore, the terms “source” and “drain” can be replaced with eachother in this specification and the like.

Note that in this specification and the like, the term “electricallyconnected” includes the case where components are connected through anobject having any electric function. There is no particular limitationon an object having any electric function as long as electric signalscan be transmitted and received between components that are connectedthrough the object.

Examples of an “object having any electric function” are a switchingelement such as a transistor, a resistor, an inductor, a capacitor, andan element with a variety of functions as well as an electrode and awiring.

Since the off-state current of a transistor including an oxidesemiconductor is extremely low, stored data can be held for an extremelylong time by using the transistor. In other words, power consumption canbe adequately reduced because refresh operation becomes unnecessary orthe frequency of refresh operation can be extremely low. Stored data canbe held for a long period even when power is not supplied (note that apotential is preferably fixed).

Further, a semiconductor device according to the present invention doesnot need high voltage for writing of data and there is no problem ofdeterioration of elements. For example, unlike a conventionalnon-volatile memory, it is not necessary to inject and extract electronsinto and from a floating gate, and thus a problem such as deteriorationof a gate insulating layer does not occur at all. In other words, thesemiconductor device according to the present invention has nolimitation on the number of times of rewriting, which is a problem of aconventional non-volatile memory, and thus has significantly improvedreliability. Furthermore, data is written depending on the on state andthe off state of the transistor, whereby high-speed operation can beeasily realized. In addition, there is no need of operation for erasingdata.

Since a transistor including a material other than an oxidesemiconductor can operate at sufficiently high speed, when this iscombined with a transistor including an oxide semiconductor, asemiconductor device can perform operation (e.g., data reading) atsufficiently high speed. Further, a transistor including a materialother than an oxide semiconductor can favorably realize a variety ofcircuits (such as a logic circuit or a driver circuit) which is requiredto operate at high speed.

Thus, a semiconductor device having a novel feature can be realized bybeing provided with both the transistor including a material other thanan oxide semiconductor (in other words, a transistor capable ofoperating at sufficiently high speed) and the transistor including anoxide semiconductor (in other words, a transistor whose off-statecurrent is sufficiently small).

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIGS. 1A1, 1A2, 1B, and 1C are circuit diagrams of a semiconductordevice;

FIG. 2 is a block diagram of a semiconductor device;

FIG. 3A and FIGS. 3B1 to 3B5 are circuit diagrams of a semiconductordevice;

FIGS. 4A and 4B are circuit diagrams of a semiconductor device;

FIGS. 5A and 5B are timing charts.

FIGS. 6A and 6B are circuit diagrams of a semiconductor device;

FIGS. 7A and 7B are timing charts;

FIG. 8 is a circuit diagram of a semiconductor device;

FIGS. 9A and 9B are timing charts;

FIG. 10 is a circuit diagram of a semiconductor device;

FIG. 11 is a timing chart;

FIG. 12 is a circuit diagram of a semiconductor device;

FIG. 13 is a timing chart;

FIG. 14 is a block diagram of a semiconductor device;

FIG. 15 is a circuit diagram of a semiconductor device;

FIG. 16 is a circuit diagram of a semiconductor device;

FIG. 17A is a cross-sectional view and FIG. 17B is a plan view of asemiconductor device;

FIGS. 18A to 18G are cross-sectional views relating to a manufacturingprocess of an SOI substrate;

FIGS. 19A to 19E are cross-sectional views relating to a manufacturingprocess of a semiconductor device;

FIGS. 20A to 20D are cross-sectional views relating to a manufacturingprocess of a semiconductor device;

FIGS. 21A to 21D are cross-sectional views relating to a manufacturingprocess of a semiconductor device;

FIGS. 22A to 22C are cross-sectional views relating to a manufacturingprocess of a semiconductor device;

FIGS. 23A to 23F are diagrams of electronic appliances;

FIGS. 24A and 24B are cross-sectional view of semiconductor devices;

FIGS. 25A to 25C are cross-sectional views relating to a manufacturingprocess of a semiconductor device;

FIGS. 26A to 26C are cross-sectional views of semiconductor devices;

FIGS. 27A to 27E each illustrate a structure of an oxide material;

FIGS. 28A to 28C illustrate a structure of an oxide material;

FIGS. 29A to 29C illustrate a structure of an oxide material;

FIG. 30 is a graph showing the gate voltage dependence of mobilityobtained from a calculation;

FIGS. 31A to 31C are graphs showing the gate voltage dependence of adrain current and mobility obtained by calculation;

FIGS. 32A to 32C are graphs showing the gate voltage dependence of adrain current and mobility obtained by calculation;

FIGS. 33A to 33C are graphs showing the gate voltage dependence of adrain current and mobility obtained by calculation;

FIGS. 34A and 34B are diagrams each illustrating a cross-sectionalstructure of a transistor used in calculation;

FIGS. 35A to 35C are graphs each showing the characteristics of atransistor;

FIGS. 36A and 36B are graphs each showing the characteristics of atransistor;

FIGS. 37A and 37B are graphs each showing the characteristics of atransistor;

FIG. 38 is a graph showing the characteristics of a transistor;

FIGS. 39A and 39B are graphs showing the characteristics of atransistor;

FIG. 40 is a graph showing XRD spectra of oxide materials;

FIG. 41 is a graph showing the characteristics of a transistor;

FIG. 42A is a plan view and FIG. 42B is a cross-sectional view of asemiconductor device; and

FIG. 43A is a plan view and FIG. 43B is a cross-sectional view of asemiconductor device.

DETAILED DESCRIPTION OF THE INVENTION

Hereinafter, embodiments of the present invention will be described withreference to the drawings. Note that the present invention is notlimited to the following description and it will be readily appreciatedby those skilled in the art that modes and details can be modified invarious ways without departing from the spirit and the scope of thepresent invention. Therefore, the present invention should not beconstrued as being limited to the description in the followingembodiments.

Note that the position, the size, the range, or the like of eachstructure illustrated in drawings and the like is not accuratelyrepresented in some cases for easy understanding. For this reason, thepresent invention is not necessarily limited to the position, size,range, or the like as disclosed in the drawings and the like.

In this specification and the like, ordinal numbers such as “first”,“second”, and “third” are used in order to avoid confusion amongcomponents, and the terms do not mean limitation of the number ofcomponents.

[Embodiment 1]

In this embodiment, a basic circuit configuration and operation of asemiconductor device according to one embodiment of the presentinvention will be described with reference to FIGS. 1A1, 1A2, 1B, and1C. Note that in each of circuit diagrams, in some cases, “OS” iswritten beside a transistor in order to indicate that the transistorincludes an oxide semiconductor.

<Basic Circuit>

First, the most basic circuit configuration and its operation will bedescribed with reference to FIGS. 1A1, 1A2, 1B, and 1C. In asemiconductor device illustrated in FIG. 1A1, a bit line BL, a sourceelectrode (or a drain electrode) of a transistor 160, and a sourceelectrode (or a drain electrode) of a transistor 162 are electricallyconnected to each other. A source line SL is electrically connected tothe drain electrode (or the source electrode) of the transistor 160. Agate line GL is electrically connected to a gate electrode of thetransistor 162. A gate electrode of the transistor 160 and the drainelectrode (or the source electrode) of the transistor 162 areelectrically connected to one electrode of a capacitor 164. A capacitorline CL is electrically connected to the other electrode of thecapacitor 164. Note that a structure may be employed in which the sourceelectrode (or the drain electrode) of the transistor 160 and the sourceelectrode (or the drain electrode) of the transistor 162 may beconnected to different wirings respectively so as not to be electricallyconnected to each other.

Here, a transistor including an oxide semiconductor is used as thetransistor 162, for example. A transistor including an oxidesemiconductor has a characteristic of an extremely low off-statecurrent. For that reason, a potential of the gate electrode of thetransistor 160 can be held for an extremely long time by turning off thetransistor 162. Provision of the capacitor 164 facilitates holding ofcharge given to the gate electrode of the transistor 160 and reading ofstored data.

Note that there is no particular limitation on a semiconductor materialof the transistor 160. In terms of increasing the speed of reading data,it is preferable to use, for example, a transistor with high switchingrate such as a transistor using single crystal silicon. The cases wherea p-channel transistor is used as the transistor 160 are illustrated inFIGS. 1A1, 1A2, and 1B. The case where an n-channel transistor is usedas the transistor 160 is illustrated in FIG. 1C.

Alternatively, the capacitor 164 can be omitted as in FIG. 1B.

The semiconductor device illustrated in FIG. 1A1 utilizes an advantagethat a potential of the gate electrode of the transistor 160 can beheld, thereby writing, holding, and reading data as follows.

Firstly, writing and holding of data will be described. First, thepotential of the gate line GL is set to a potential which allows thetransistor 162 to be turned on, so that the transistor 162 is turned on.Thus, the potential of the bit line BL is supplied to a node (alsoreferred to as a floating gate portion FG) to which the drain electrode(or the source electrode) of the transistor 162, the gate electrode ofthe transistor 160, and the one electrode of the capacitor 164 areelectrically connected. In other words, predetermined charge is suppliedto the floating gate portion FG (writing). Here, any one of chargessupplying two different potentials (hereinafter a charge supplying a lowpotential is referred to as a charge Q_(L) and a charge supplying a highpotential is referred to as a charge Q_(H)) is given. Note that chargesfor supplying three or more different potentials may be applied toimprove a storage capacitor. After that, the potential of the gate lineGL is set to a potential which allows the transistor 162 to be turnedoff, so that the transistor 162 is turned off. Thus, the charge suppliedto the floating gate portion FG is held (holding).

Since the off-state current of the transistor 162 is extremely low, thecharge of the gate electrode of the transistor 160 is held for a longtime.

Secondly, reading of data will be described. An appropriate potential (areading potential) is supplied to the capacitor line CL in the statewhere a predetermined potential (a fixed potential) is supplied to thesource line SL, whereby the potential of the bit line BL varies inresponse to the amount of charge held in the floating gate portion FG.In other words, the conductance of the transistor 160 is controlled bythe charge held in the gate electrode (which can also be referred to asthe floating gate portion FG) of the transistor 160.

In general, when the transistor 160 is a p-channel transistor, anapparent threshold voltage V_(th) _(—) _(H) in the case where Q_(H) issupplied to the gate electrode of the transistor 160 is lower than anapparent threshold voltage V_(th) _(—) _(L) in the case where Q_(L) issupplied to the gate electrode of the transistor 160. For example, inthe case where Q_(L) is supplied in writing, when the potential of thecapacitor line CL is V₀ (a potential intermediate between V_(th) _(—)_(H) and V_(th) _(—) _(L)), the transistor 160 is turned on. In the casewhere Q_(H) is supplied in writing, even when the potential of thecapacitor line CL is V₀, the transistor 160 remains off. Thus, the dataheld can be read by detecting the potential of the bit line BL.

Thirdly, rewriting of data will be described. Rewriting of data isperformed in a manner similar to that of the writing and holding ofdata. In other words, the potential of the gate line GL is set to apotential which allows the transistor 162 to be turned on, so that thetransistor 162 is turned on. Thus, the potential of the bit line BL (apotential related to new data) is supplied to the floating gate portionFG. Then, the potential of the gate line GL is set to a potential atwhich the transistor 162 is turned off, whereby the transistor 162 isturned off. Consequently, a charge related to new data is supplied toand kept in the floating gate portion FG.

In the semiconductor device according to one embodiment of the presentinvention, data can be directly rewritten by another writing of data asdescribed above. For that reason, extracting of charge from a floatinggate with the use of a high voltage needed in a flash memory or the likeis not necessary and thus reduction in operation speed, which isattributed to erasing operation, can be suppressed. In other words,high-speed operation of the semiconductor device can be realized.

As an example, a method for writing, holding, and reading in the casewhere a potential VDD or a ground potential GND is supplied to thefloating gate portion FG is specifically described below. In thefollowing description, data that is held when the potential VDD issupplied to the floating gate portion FG is referred to as data “1”, anddata that is held when the ground potential GND is supplied to thefloating gate portion FG is referred to as data “0”. Note that therelation between the potentials supplied to the floating gate portion FGis not limited to this.

When data is written, the source line SL is set to GND, the capacitorline CL is set to GND, and the gate line GL is set to VDD, so that thetransistor 162 is turned on. When data “0” is written to the floatinggate portion FG, GND is supplied to the bit line BL. When data “1” iswritten to the floating gate portion FG, the potential of the bit lineBL may be set to VDD and the potential of the gate line GL may be set toVDD+Vth_OS so that the potential of the floating gate portion FG is notlowered by the same amount as the threshold voltage (Vth_OS) of thetransistor 162.

When data is held, the gate line GL is set to GND, so that thetransistor 162 is turned off. In order to reduce power consumption dueto a current generated in the bit line BL and the source line SL throughthe transistor 160 that is a p-channel transistor, the bit line BL andthe source line SL are set to the same potential. Note that thepotential of the capacitor line CL may be either VDD or GND as long asthe potential of the bit line BL and the potential of the source line SLare the same.

Note that the above expression “the same potential” includes“approximately the same potential”. In other words, an object of theabove is to reduce a current generated in the bit line BL and the sourceline SL by adequately reducing the potential difference between the bitline BL and the source line SL; therefore, “approximately the samepotential”, e.g., a potential which enables power consumption to besufficiently reduced (to one hundredth or less) compared to the casewhere the potential of the source line SL is fixed to GND or the like,is included in “the same potential”. In addition, potential deviationdue to wire resistance or the like are reasonably acceptable.

When data is read, the potential of the gate line GL is set to GND, thepotential of the capacitor line CL is set to GND, and the potential ofthe source line SL is set to VDD or a potential slightly lower than VDD(hereinafter referred to as VR). Here, in the case where data “1” iswritten to the floating gate portion FG, the transistor 160 that is ap-channel transistor is turned off and the potential of the bit line BLat the beginning of the reading is maintained or is raised. Note that itdepends on a reading circuit connected to the bit line BL whether thepotential of the bit line BL is maintained or raised. In the case wheredata “0” is written to the floating gate portion FG, the transistor 160is turned on and the potential of the bit line BL is set at VDD or VRwhich is the same potential as that of the source line SL. Thus, thedata “1” or the data “0” which is held in the floating gate portion FGcan be read depending on the potential of the bit line BL.

Note that in the case where the potential VDD is held in (that is, data“1” is written to) the floating gate portion FG, the potential of thesource line SL is set to VDD at the time of reading, so that a voltagebetween the gate and the source of the transistor 160 (hereinafterreferred to as Vgsp) is set at Vgsp=VDD−VDD=0 V and Vgsp is set higherthan the threshold voltage of the transistor 160 (hereinafter referredto as Vthp); thus, the transistor 160 that is a p-channel transistor isturned off. Here, even in the case where a potential held in thefloating gate portion FG is lower than VDD because a potential writtento the floating gate portion FG is lower than VDD, the transistor 160 isturned off because the relation of Vgsp=(VDD—|Vthp|)−VDD=−|Vthp|=Vthp issatisfied when the potential of the floating gate portion FG is higherthan or equal to VDD−|Vthp|; thus, data “1” can be read accurately.However, in the case where the potential of the floating gate portion FGis lower than VDD−|Vthp|, the transistor 160 is turned on because Vgspis set lower than Vthp; thus, not data “1” but data “0” is read,resulting in misreading. In other words, in the case where data “1” iswritten, the lower limit of a potential at which data can be read islower than the potential VDD of the source line SL by |Vthp|, that is,VDD−|Vthp|. On the other hand, when the potential of the source line SLis set to VR at the time of reading, the lower limit of a potential atwhich data “1” can be read is lower than the potential VR of the sourceline SL by |Vthp|, that is VR−|Vthp| as described above. Here, since thepotential VR is lower than the potential VDD, VR−|Vthp| is lower thanVDD−|Vthp|. In other words, the lower limit of the potential at whichdata “1” can be read is lowered when the potential of the source line SLis set to VR. Consequently, VR is preferable to VDD as the potential ofthe source line SL because a potential range which data “1” can be readcan be wide. As for the highest potential at which data can be read out,in the case where the potential of the source line SL is set to VR, Vgspbecomes VDD−VR>Vthp (because of VDD>VR) when VDD is written to thefloating gate portion FG, so that the transistor 160 can be turned offwithout problems.

Here, the node (the floating gate portion FG) to which the drainelectrode (or the source electrode) of the transistor 162, the gateelectrode of the transistor 160, and the one electrode of the capacitor164 are electrically connected has an effect similar to that of afloating gate of a floating-gate transistor which is used as anon-volatile memory element. When the transistor 162 is off, thefloating gate portion FG can be regarded as being embedded in aninsulator and thus charge is held in the floating gate portion FG. Theoff-state current of the transistor 162 including an oxide semiconductoris less than or equal to 1/100,000 of the off-state current of atransistor including a silicon semiconductor or the like; thus, loss ofthe charge accumulated in the floating gate portion FG due to leakagecurrent of the transistor 162 is negligible. That is, with thetransistor 162 including an oxide semiconductor, a non-volatile memorydevice which can hold data without being supplied with power can berealized.

For example, when the off-state current of the transistor 162 is lessthan or equal to 10 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) at roomtemperature (25° C.) and the capacitance value of the capacitor 164 isapproximately 10 fF, data can be held for 10⁴ seconds or longer. It isneedless to say that the holding time depends on transistorcharacteristics and the capacitance value.

Further, in the semiconductor device according to one embodiment of thepresent invention, the problem of deterioration of a gate insulatingfilm (tunnel insulating film), which is pointed out in a conventionalfloating gate transistor, does not exist. That is, the problem ofdeterioration of a gate insulating film due to injection of electronsinto a floating gate, which has been regarded as a problem, can besolved. This means that there is no limit on the number of times ofwriting in principle. Furthermore, a high voltage needed for writing orerasing in a conventional floating gate transistor is not necessary.

The components such as transistors in the semiconductor device in FIG.1A1 can be regarded as including a resistor and a capacitor as shown inFIG. 1A2. That is, in FIG. 1A2, the transistor 160 and the capacitor 164are each regarded as including a resistor and a capacitor. R1 and C1denote the resistance value and the capacitance value of the capacitor164, respectively. The resistance R1 corresponds to the resistance of aninsulating layer included in the capacitor 164. R2 and C2 denote theresistance value and the capacitance value of the transistor 160,respectively. The resistance value R2 corresponds to the resistancevalue which depends on a gate insulating layer at the time when thetransistor 160 is on. The capacitance C2 corresponds to so-called gatecapacitance (capacitance formed between the gate electrode and thesource electrode or the drain electrode and capacitance formed betweenthe gate electrode and the channel formation region).

A charge holding period (also referred to as a data holding period) isdetermined mainly by the off-state current of the transistor 162 underthe conditions where the gate leakage current of the transistor 162 issufficiently small and R1 and R2 satisfy R1≧ROS and R2≧ROS, where ROS isthe resistance (also referred to as effective resistance) between thesource electrode and the drain electrode in a state where the transistor162 is turned off.

On the other hand, when the conditions are not satisfied, it isdifficult to sufficiently secure the holding period even if theoff-state current of the transistor 162 is low enough. This is because aleakage current other than the off-state current of the transistor 162(e.g., a leakage current generated between the source electrode and thegate electrode) is high. Thus, it can be said that the semiconductordevice according to one embodiment of the present invention desirablysatisfies the relation where R1≧ROS and R2≧ROS.

On the other hand, it is desirable that C1≧C2 be satisfied. This isbecause if the capacitance C1 is large, the potential of the capacitorline CL can be supplied to the floating gate portion FG efficiently atthe time of controlling the potential of the floating gate portion FG bythe capacitor line CL, and a difference between potentials (e.g., areading potential and a non-reading potential) supplied to the capacitorline CL can be made small.

As described above, when the above relation is satisfied, a morefavorable semiconductor device can be realized. Note that R1 and R2 arecontrolled by the gate insulating layer of the transistor 160 and theinsulating layer of the capacitor 164. The same relation is applied toC1 and C2. Therefore, the material, the thickness, and the like of thegate insulating layer are desirably set as appropriate to satisfy theabove relation.

In the semiconductor device described in this embodiment, the floatinggate portion FG has an effect similar to a floating gate of afloating-gate transistor in a flash memory or the like, but the floatinggate portion FG of this embodiment has a feature which is essentiallydifferent from that of the floating gate in the flash memory or thelike.

In a flash memory, since a potential applied to a control gate is high,it is necessary to keep a proper distance between cells in order toprevent the potential from affecting a floating gate of the adjacentcell. This is one of inhibiting factors for high integration of thesemiconductor device. The factor is attributed to a basic principle of aflash memory, in which a tunneling current flows in applying a highelectrical field.

In contrast, the semiconductor device according to this embodiment isoperated by switching of a transistor including an oxide semiconductorand does not use the above-described principle of charge injection bytunneling current. That is, a high electrical field for charge injectionis not necessary unlike a flash memory. Consequently, it is notnecessary to consider an influence of a high electrical field from acontrol gate on an adjacent cell, which facilitates high integration.

In addition, it is also advantage over a flash memory that a highelectric field is unnecessary and a large peripheral circuit (such as abooster circuit) is unnecessary. For example, the highest voltageapplied to the memory cell according to this embodiment (the differencebetween the highest potential and the lowest potential applied toterminals of the memory cell at the same time) can be lower than orequal to 5 V, preferably lower than or equal to 3 V in each memory cellin the case where two levels (one bit) of data are written.

In the case where the relative permittivity ∈r1 of the insulating layerincluded in the capacitor 164 is different from the relativepermittivity ∈r2 of the insulating layer included in the firsttransistor 160, it is easy to satisfy C1≧C2 while satisfying 2·S2≧S1(desirably S2≧S1), where S1 is the area of the insulating layer includedin the capacitor 164 and S2 is the area of the insulating layer forminga gate capacitor of the transistor 160. In other words, C1 can easily bemade greater than or equal to C2 while the area of the insulating layerincluded in the capacitor 164 is made small. Specifically, for example,a film including a high-k material such as hafnium oxide or a stack of afilm including a high-k material such as hafnium oxide and a filmincluding an oxide semiconductor is used for the insulating layerincluded in the capacitor 164 so that ∈r1 can be set to 10 or more,preferably 15 or more, and silicon oxide is used for the insulatinglayer forming the gate capacitor so that ∈r2 can be set to 3 to 4.

A combination of such structures enables still higher integration of thesemiconductor device according to one embodiment of the presentinvention.

<Application Example>

Next, a more specific circuit configuration to which the circuitillustrated in FIGS. 1A1, 1A2, 1B, and 1C is applied and an operationthereof will be described with reference to FIG. 2, FIGS. 3A and 3B1 to3B5, FIGS. 4A and 4B, FIGS. 5A and 5B, FIGS. 6A and 6B, FIGS. 7A and 7B,FIG. 8, FIGS. 9A and 9B, FIG. 10, FIG. 11, FIG. 12, FIG. 13, FIG. 14,FIG. 15, and FIG. 16. In this embodiment, a so-called multi-valuedmemory which holds a plurality of states in one memory cell is described

FIG. 2 is an example of a block diagram of a semiconductor device. Afeature of a block diagram illustrated in FIG. 2 relates to writingoperation of a driver circuit. A semiconductor device illustrated inFIG. 2 is a multi-valued memory which holds 2^(k)-valued (k is aninteger greater than or equal to 1) state in one memory cell andincludes a memory cell array 201 including a plurality of memory cells,a column driver circuit 202, and a row driver circuit 203.

The memory cell array 201 includes m gate lines GL and m capacitor linesCL, n bit lines BL, a source line SL (not shown in FIG. 2), and aplurality of memory cells 170(1,1) to 170(m, n) arranged in a matrix.

As the memory cells 170(1, 1) to 170(m, n) illustrated in FIG. 2, thememory cell illustrated in FIG. 1A1 can be applied. Alternatively, asthe memory cells 170(1, 1) to 170(m, n), the memory cell illustrated inFIG. 1B can be applied. In that case, the capacitor lines CL can beomitted. Further alternatively, as the memory cells 170(1, 1) to 170(m,n), the memory cell illustrated in FIG. 1C can be applied.

As the structure of the memory cell array 201, either a structure ofFIG. 15 or a structure of FIG. 16 can be used.

FIG. 15 illustrates an example of the memory cell array. The memory cellarray illustrated in FIG. 15 is an NOR memory cell array and includes mgate lines GL, m capacitor lines CL, n bit lines BL, (n/8) source linesSL, and a plurality of memory cells 170(1,1) to 170(m, n). Here, thememory cells 170 are arranged in a matrix of m (rows) (in a verticaldirection) by n (columns) (in a horizontal direction). Here, one sourceline SL is provided for eight columns of the memory cells 170.Accordingly, the number of wirings can be small as compared to that inthe case where one source line SL is provided for every column. Inaddition, the space of the memory cell array 201 can be saved. Needlessto say, n source lines SL may be provided in the memory cell array 201illustrated in FIG. 15. A pre-charge potential PRE is input to thesource lines SL(1) to SL(n/8) through buffers 208.

The n bit lines BL and the (n/8) source lines SL are connected to a bitline/source line driver circuit 221 included in the column drivercircuit 202 illustrated in FIG. 2. In addition, the m gate lines GL andthe m capacitor lines CL are connected to a gate line/capacitor linedriver circuit 231 included in the row driver circuit 203 illustrated inFIG. 2.

FIG. 16 illustrates another example of the memory cell array. The memorycell array illustrated in FIG. 16 is a NAND memory cell array andincludes one selection line G(1), m gate lines GL, m capacitor lines CL,n bit lines BL, one source line SL, and a plurality of memory cells 170.Here, the memory cells 170 are arranged in a matrix of n7 (rows) (in avertical direction) by n (columns) (in a horizontal direction).

The n bit lines BL and the one source line SL are connected to the bitline/source line driver circuit 221 included in the column drivercircuit 202 illustrated in FIG. 2. In addition, the one selection lineG(1), the in gate lines GL, and the m capacitor lines CL are connectedto the gate line/capacitor line driver circuit 231 included in the rowdriver circuit 203 illustrated in FIG. 2.

Column address signal lines CA, input data signal lines DIN, output datasignal lines DOUT, a control signal line CE, and the like are connectedto the column driver circuit 202 illustrated in FIG. 2. In addition, inthe column driver circuit 202, reading circuits 225(1) to 225(n) for therespective columns of the memory cells 170 are provided. The readingcircuits 225(1) to 225(n) are connected to the memory cells 170 throughthe bit lines BL(1) to BL(n), respectively. The column driver circuit202 controls the bit lines BL(1) to BL(n) and the source lines SL.

A row address signal line RA, the control signal line CE, and the likeare connected to the row driver circuit 203. In addition, the row drivercircuit 203 is connected to the memory cells 170 through the gate linesGL and the capacitor lines CL. The row driver circuit 203 controls theselection line G, the gate lines GL, and the capacitor lines CL.

<Reading Circuit>

Next, a reading circuit which can be applied to FIG. 2 and a drivingmethod thereof are described.

FIG. 3A illustrates an example of the reading circuit. The readingcircuit illustrated in FIG. 3A includes a load 323 and a sense amplifier324. The load 323 and the bit line BL are connected to an input of thesense amplifier 324, and an output signal line SA_OUT is connected to anoutput of the sense amplifier 324. In addition, the memory cell 170 isconnected to the bit line.

As the load 323, any of FIGS. 3B1, 3B2, 3B3, 3B4, and 3B5 can be used.As illustrated in FIG. 3B1, the load 323 may have a structure in which aconstant-voltage power supply line Vread is connected to a gate terminalof an n-channel transistor. As illustrated in FIG. 3B2, the load 323 maybe a resistor. Alternatively, as illustrated in FIG. 3B3, theconstant-voltage power supply line Vread may be connected to a gateterminal of a p-channel transistor. Further alternatively, asillustrated in FIG. 3B4, the load 323 may have a structure in which agate terminal of an n-channel transistor is connected to one of a sourceterminal or a drain terminal. Further alternatively, as illustrated inFIG. 3B5, the load 323 may have a structure in which a gate terminal ofa p-channel transistor is connected to one of a source terminal and adrain terminal.

The potential of the bit line BL is determined by the resistancedivision of the load 323 and a reading transistor in the memory cell.The output of the sense amplifier 324 is changed in response to thechange of the potential of the bit line BL.

As a specific circuit example of the sense amplifier 324, an invertercan be given. The inverter is configured so that an output signalthereof changes at the time when the potential of an input signalbecomes a half of a power supply potential VDD. In addition, with theuse of the inverter, the number of circuit components becomes small,whereby a space-saving reading circuit can be achieved.

FIG. 4A is a circuit diagram of an inverter 325. The inverter in FIG. 4Aincludes a p-channel transistor 341 and an n-channel transistor 342. Thetransistor 341 is connected to the transistor 342 in series.Specifically, a gate terminal of the transistor 341, a gate terminal ofthe transistor 342, and an input terminal are connected to each other.In addition, a drain terminal (or a source terminal) of the transistor341, a source terminal (or a drain terminal) of the transistor 342, andan output terminal OUT are connected to each other. The source terminal(or the drain terminal) of the transistor 341 is connected to VDD. Thedrain terminal (or the source terminal) of the transistor 342 isgrounded to GND.

FIG. 4B illustrates a reading circuit with the use of the inverter 325as the sense amplifier 324 illustrated in FIG. 3A, an analog switch 223a, and the memory cell 170. A bit line BL of the memory cell 170 isconnected to one terminal of the load 323 and an input terminal of theinverter 325 of the reading circuit through the analog switch 223 a. Theother terminal of the load 323 of the reading circuit is grounded to aground potential GND or connected to a power supply potential VDD. Anoutput terminal of the inverter 325 is connected to an output signalline SA_OUT. The bit line BL of the memory cell 170 is connected to adrain terminal of the transistor 160 and one of a drain terminal and asource terminal of the transistor 162. A source terminal of thetransistor 160 is connected to the power supply potential VDD orgrounded to the ground potential GND.

Here, the transistor 160 of the memory cell 170 is a p-channeltransistor, and the potential of a source line SL connected to thep-channel transistor is the power supply potential VDD. In addition, thepotential of the other terminal of the load 323 of the reading circuitis the ground potential GND.

Next, FIGS. 5A and 5B show timing charts in reading operation of FIG.4B.

In the reading operation, the potential of the capacitor line CL of thememory cell 170 is changed, and the potential of the floating gateportion FG is also changed by capacitive coupling. The resistance valueof the transistor 160 of the memory cell 170 is changed in response tothe change of the potential of the floating gate portion FG.

The potential of the bit line BL is determined by the resistancedivision of the resistance value of the transistor 160 of the memorycell 170 and the load 323 in the reading circuit. Sensing of thepotential of the bit line BL is performed by the inverter 325, wherebyreading of multi-valued data can be realized.

Specifically, as shown in FIG. 5A, in the case where the potential ofthe capacitor line CL is gradually decreased from High potential(hereinafter referred to H potential) to Low potential (hereinafterreferred to as L potential), the potential of the floating gate portionFG is gradually decreased from the high potential side to the lowpotential side by capacitive coupling in a similar manner to thecapacitor line CL. When the potential of the floating gate portion FG isgradually decreased from the high potential side to the low potentialside, the resistance value of the reading transistor (the transistor 160in FIG. 4B) is changed from the high resistance side to the lowresistance side. Since the potential of the bit line BL is determined bythe resistance division of the load 323 in the reading circuit and thetransistor 160, as the resistance value of the transistor 160 isdecreased, the potential of the bit line BL is raised.

When the potential of the bit line BL exceeds a certain potential (e.g.,(VDD/2)), the potential output from the inverter 325 of the readingcircuit to the output signal line SA_OUT is changed from H potential toL potential. Data of the memory cell 170 can be determined depending onthe position where the potential is changed.

Here, as the potential of the bit line BL gets closer to (VDD/2), theamount of a through current of the inverter 325 becomes larger.Specifically, in the case where the potential of the bit line BL is(VDD/2), a potential difference of (VDD/2) is generated between the gateterminal and the drain terminal of the p-channel transistor 341, and thesource terminal and the drain terminal of the transistor 341 are broughtinto conduction. In a similar manner, a potential difference of (VDD/2)is generated between the gate terminal and the source terminal of then-channel transistor 342, and the source terminal and the drain terminalof the n-channel transistor 342 are brought into conduction. Thus, thepower supply potential VDD and the ground potential GND are brought intoconduction through the inverter 325, whereby a through current isgenerated. When the potential of the bit line BL varies from (VDD/2),the resistance value of the p-channel transistor 341 or the resistancevalue of the n-channel transistor 342 is raised and the through currentis gradually suppressed. Note that a current I_INV in FIG. 5A shows theamount of the through current of the inverter 325.

In a similar manner, in FIG. 5B, when the potential of the capacitorline CL is gradually raised from L potential to H potential, thepotential of the floating gate portion FG is gradually raised from thelow potential side to the high potential side by capacitive coupling ina similar manner to the capacitor line CL. When the potential of thefloating gate portion FG is gradually raised from the low potential sideto the high potential side, the resistance value of the transistor 160is changed from the low resistance side to the high resistance side. Asthe resistance value of the transistor 160 is raised, the potential ofthe bit line BL is decreased because the potential of the bit line BL isdetermined by the resistance division of the load 323 in the readingcircuit and the transistor 160.

When the potential of the bit line BL exceeds a certain potential (e.g.,(VDD/2)), the potential output from the inverter 325 of the readingcircuit to the output signal line SA_OUT is changed from L potential toH potential. The data of the memory cell 170 can be read from theposition where the potential is changed. Note that a current I_INV ofthe FIG. 5B shows the amount of the through current of the inverter 325.

The inverter 325 is used as the specific circuit example of the senseamplifier 324, whereby the size of the memory reading circuit can bereduced. Accordingly, the space of a memory peripheral circuit can besaved.

Next, other examples of a reading circuit which are different from FIGS.4A and 4B are described with reference to FIGS. 6A and 6B and FIG. 8. Ineach of FIGS. 6A and 6B and FIG. 8, a clocked inverter 326 controlled byclock signals is used as a sense amplifier. In order to prevent a highimpedance state of the output of the clocked inverter 326, a p-channeltransistor 327 or an n-channel transistor 328 connected to VDD or GND isconnected to an output terminal of the clocked inverter 326.

FIG. 6A illustrates a circuit diagram of the clocked inverter 326. Theclocked inverter 326 includes two p-channel transistors and twon-channel transistors, which are sequentially connected in series. Thep-channel transistor 341 and the n-channel transistor 342 are connectedin series. Specifically, the gate terminal of the transistor 341, thegate terminal of the transistor 342, and an input terminal IN areconnected to each other. Further, the drain terminal (or the sourceterminal) of the transistor 341, the source terminal (or the drainterminal) of the transistor 342, and the output terminal OUT areelectrically connected to each other. In addition, the source terminal(or the drain terminal) of the p-channel transistor 341 and a drainterminal (or a source terminal) of a p-channel transistor 343 areconnected to each other. The transistor 343 is connected to VDD. Thedrain terminal (or the source terminal) of the n-channel transistor 342and a source terminal (or a drain terminal) of an n-channel transistor344 are connected to each other. The drain terminal (or the sourceterminal) of the transistor 344 is grounded to GND. An inversion controlsignal line CLKB (referred to as a CLKB signal line) is connected to agate terminal of the transistor 343 and a control signal line CLK(referred to as a CLK signal line) is connected to a gate terminal ofthe transistor 344. Note that the inversion control signal line CLKB isan inversion signal of the control signal line CLK.

In the case where the potential of the control signal line CLK is Hpotential and the potential of the inversion control signal line CLKB isL potential, when the potential of an input signal line is (VDD/2) ormore, the clocked inverter 326 outputs L potential to an output signalline; when the potential of the input signal line is less than (VDD/2),the clocked inverter 326 outputs H potential to the output signal line.In addition, in the case where the potential of the control signal lineCLK is L potential and the potential of the inversion control signalline CLKB is H potential, the output signal line is in a high impedancestate regardless of the potential of the bit line BL.

FIG. 6B illustrates a reading circuit in which the load 323 is connectedto the input of the clocked inverter 326, and the p-channel transistor327 connected to VDD is connected to the output of the clocked inverter326. In addition, the reading circuit is connected to the memory cell170 through the analog switch 223 a.

Next, FIGS. 7A and 7B show timing charts in reading operation of FIGS.6A and 6B. FIG. 7A shows a timing chart in the case of FIG. 6B in whichthe potential of the capacitor line CL is gradually decreased from Hpotential to L potential, and FIG. 7B shows a timing chart in the caseof FIG. 6B in which the potential of the capacitor line CL is graduallyraised from L potential to H potential.

In FIG. 7A, in the case where the potential of the capacitor line CL isgradually decreased from H potential to L potential, the potential ofthe floating gate portion FG is gradually decreased from the highpotential side to the low potential side by capacitive coupling in asimilar manner to the capacitor line CL. When the potential of thefloating gate portion FG is decreased from the high potential side tothe low potential side, the resistance value of the transistor 160 ischanged from the high resistance side to the low resistance side. As theresistance value of the transistor 160 is decreased, the potential ofthe bit line BL is raised because the potential of the bit line BL isdetermined by the resistance division of the load 323 in the readingcircuit and the transistor 160.

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate an outputsignal corresponding to the potential of the bit line BL in the outputsignal line SA_OUT whenever the potential of the bit line BL is changed.

Specifically, in the case where the potential of the CLK signal is Hpotential, since H potential is input to a gate terminal of thetransistor 327 connected to the power supply potential VDD, the powersupply potential VDD and the output signal line SA_OUT are brought outof conduction, and the output signal line SA_OUT reflects the output ofthe clocked inverter 326 with respect to the potential of the bit lineBL. When the potential of the bit line BL exceeds a certain potential(e.g., (VDD/2)), the potential output from the clocked inverter 326 ofthe reading circuit to the output signal line SA_OUT is changed from Hpotential to L potential. When the potential of the CLK signal is Lpotential, the clocked inverter 326 is in a high impedance state withrespect to the output signal line SA_OUT, and the power supply potentialVDD and the output signal line SA_OUT are brought into conduction by thep-channel transistor 327 connected to the power supply potential VDD.Therefore, the output signal line SA_OUT is at H potential regardless ofthe potential of the bit line BL.

Here, in the case where the potential of the CLK signal is H potential,as the potential of the bit line BL gets closer to (VDD/2), the amountof a through current of the clocked inverter 326 becomes larger.Specifically, in the case where the potential of the bit line BL is(VDD/2) and L potential is input to the gate terminal of the transistor343 of the clocked inverter, the power supply potential VDD and thedrain terminal of the transistor 341 are brought into conduction. Atthat time, a potential difference of (VDD/2) is generated between thegate terminal and the drain terminal of the transistor 341 of theclocked inverter, and the source terminal and the drain terminal of thetransistor 341 are brought into conduction. In a similar manner, Hpotential is input to the gate terminal of the transistor 344 of theclocked inverter, and the ground potential GND and the source terminalof the transistor 342 are brought into conduction. At that time, apotential difference of (VDD/2) is generated between the gate terminaland the drain terminal of the transistor 342, and the source terminaland the drain terminal of the transistor 342 are brought intoconduction. Then, the power supply potential VDD and the groundpotential GND are brought into conduction through the clocked inverter326, so that a through current is generated. When the potential of thebit line BL varies from (VDD/2), the resistance value of the transistor341 or the resistance value of the transistor 342 is raised and thethrough current is gradually suppressed.

However, when the potential of the CLK signal is L potential, Hpotential is input to the gate terminal of the transistor 343, and thepower supply potential VDD and the drain terminal of the transistor 341are brought out of conduction. In addition, L potential is input to thegate terminal of the transistor 344, and the ground potential GND andthe source terminal or the transistor 342 are brought out of conduction.Therefore, since the power supply potential VDD and the ground potentialGND are not brought into conduction through the clocked inverter 326,the through current flowing through the clocked inverter 326 issuppressed regardless of the potential of the bit line BL. Note that acurrent I_INV of FIG. 7A shows the amount of the through current of theclocked inverter 326. In the case where the clocked inverter 326 is usedas a sense amplifier as illustrated in FIG. 7A, the through current canbe further suppressed as compared to the case where the inverter 325 isused as a sense amplifier illustrated in FIG. 5A.

The data of the memory cell 170 can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling a time in which the potentialof the CLK signal is H potential, the through current of the clockedinverter 326 which is consumed in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. As described above, the time in which the potential ofthe CLK signal is H potential is preferably shorter because a time inwhich the through current of the clocked inverter 326 is being generatedcan be shortened, which is effective in the reduction of currentconsumption of the reading circuit.

Next, in FIG. 7B, when the potential of the capacitor line CL isgradually raised from L potential to H potential, the potential of thefloating gate portion FG is gradually raised from the low potential sideto the high potential side by capacitive coupling in a similar manner tothe capacitor line CL. When the potential of the floating gate portionFG is raised from the low potential side to the high potential side, theresistance value of the transistor 160 is changed from the highresistance side to the low resistance side. As the resistance value ofthe transistor 160 is raised, the potential of the bit line BL isdecreased because the potential of the bit line BL is determined by theresistance division of the load 323 in the reading circuit and thetransistor 160.

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate an outputsignal corresponding to the potential of the bit line BL in the outputsignal line SA_OUT whenever the potential of the bit line BL is changed.

Specifically, in the case where the potential of the CLK signal is Hpotential, since H potential is input to the gate terminal of thetransistor 327 connected to the power supply potential VDD, the powersupply potential VDD and the output signal line SA_OUT are brought outof conduction, and the output signal line SA_OUT reflects the output ofthe clocked inverter 326 with respect to the potential of the bit lineBL. When the potential of the bit line BL is less than a certainpotential (e.g., (VDD/2)), the potential output from the clockedinverter 326 of the reading circuit to the output signal line SA_OUT ischanged from H potential to L potential. When the potential of the CLKsignal is L potential, the clocked inverter 326 is in a high impedancestate with respect to the output signal line SA_OUT, and VDD and theoutput signal line SA_OUT are brought into conduction by the p-channeltransistor 327 connected to VDD. Therefore, the output signal lineSA_OUT is at H potential regardless of the potential of the bit line BL.

As in the description of FIG. 7A, in the case where the clocked inverter326 is used as a sense amplifier as illustrated in FIG. 7B, the throughcurrent can be further suppressed as compared to the case where theinverter 325 is used as a sense amplifier as illustrated in FIG. 5B.

The data of the memory cell 170 can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling the time of reading with theCLK signal, current consumption in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. When the potential of the CLK signal is H potential fora short time as illustrated in FIG. 7B, current consumption iseffectively reduced. Note that this is based on the premise that thereis no harm in reading operation.

As described above, with the use of the clocked inverter 326 as aspecific circuit example of a sense amplifier, a memory reading circuitin which a through current is suppressed can be achieved with a smallnumber of circuit components. Accordingly, the space of the peripherycircuit of the memory cell array can be saved and current consumptioncan be reduced.

FIG. 8 illustrates a reading circuit in which the load 323 is connectedto the input of the clocked inverter 326, and the n-channel transistor328 grounded to GND is connected to the output of the clocked inverter326. The reading circuit is connected to the memory cell 170 through theanalog switch 223 a.

FIGS. 9A and 9B show timing charts in reading operation of FIG. 8. FIG.9A shows a timing chart in the case of FIG. 8 in which the potential ofthe capacitor line CL is gradually decreased from H potential to Lpotential, and FIG. 9B shows a timing chart in the case of FIG. 8 inwhich the potential of the capacitor line CL is gradually raised from Lpotential to H potential.

In FIG. 9A, when the potential of the capacitor line CL is graduallydecreased from H potential to L potential, the potential of the floatinggate portion FG is gradually decreased from the high potential side tothe low potential side by capacitive coupling in a similar manner to thecapacitor line CL. When the potential of the floating gate portion FG isdecreased from the high potential side to the low potential side, theresistance value of the transistor 160 is changed from the highresistance side to the low resistance side. As the resistance value ofthe transistor 160 is decreased, the potential of the bit line BL israised because the potential of the bit line BL is determined by theresistance division of the load 323 in the reading circuit and thetransistor 160.

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate an outputsignal corresponding to the potential of the bit line BL in the outputsignal line SA_OUT whenever the potential of the bit line BL is changed.

Specifically, in the case where the potential of a CLKB signal is Lpotential, the ground potential GND and the output signal line SA_OUTare brought out of conduction because L potential is input to a gateterminal of the transistor 328 grounded to the ground potential GND, andthe output signal line SA_OUT reflects the output of the clockedinverter 326 with respect to the potential of the bit line BL. When thepotential of the bit line BL is less than a certain potential (e.g.,(VDD/2)), the potential output from the clocked inverter 326 of thereading circuit to the output signal line SA_OUT is changed from Lpotential to H potential. When the potential of the CLKB signal is Hpotential, the clocked inverter 326 is in a high impedance state withrespect to the output signal line SA_OUT, and the ground potential GNDand the output signal line SA_OUT are brought into conduction by then-channel transistor 328 grounded to the ground potential GND.Therefore, the output signal line SA_OUT is at L potential regardless ofthe potential of the bit line BL.

As in the description of FIG. 7A, in the case where the clocked inverter326 is used as a sense amplifier as illustrated in FIG. 9A, the throughcurrent can be further suppressed as compared to the case where theinverter 325 is used as a sense amplifier as illustrated in FIG. 5A.

The data of the memory cell 170 can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling the time of reading with theCLK signal, current consumption in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. When the potential of the CLK signal is H potential fora short time as illustrated in FIG. 9A, current consumption iseffectively reduced. Note that this is based on the premise that thereis no harm in reading operation.

Next, in FIG. 9B, when the potential of the capacitor line CL isgradually raised from L potential to H potential, the potential of thefloating gate portion FG is gradually raised from the low potential sideto the high potential side by capacitive coupling in a similar manner tothe capacitor line CL. When the potential of the floating gate portionFG is raised from the low potential side to the high potential side, theresistance value of the transistor 160 is changed from the highresistance side to the low resistance side. As the resistance value ofthe transistor 160 is raised, the potential of the bit line BL isdecreased because the potential of the bit line BL is determined by theresistance division of the load 323 in the reading circuit and thetransistor 160.

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate an outputsignal corresponding to the potential of the bit line BL in the outputsignal line SA_OUT whenever the potential of the bit line BL is changed.

Specifically, in the case where the potential of the CLKB signal is Lpotential, since L potential is input to the gate terminal of thetransistor 328 grounded to the ground potential GND, the groundpotential GND and the output signal line SA_OUT are brought out ofconduction, and the output signal line SA_OUT reflects the output of theclocked inverter 326 with respect to the potential of the bit line BL.When the potential of the bit line BL exceeds a certain potential (e.g.,(VDD/2)), the potential output from the clocked inverter 326 of thereading circuit to the output signal line SA_OUT is changed from Hpotential to L potential. When the potential of the CLKB signal is Hpotential, the clocked inverter 326 is in a high impedance state withrespect to the output signal line SA_OUT, and the ground potential GNDand the output signal line SA_OUT are brought into conduction by then-channel transistor 328 grounded to the ground potential GND.Therefore, the output signal line SA_OUT is at L potential regardless ofthe potential of the bit line BL.

As in the description of FIG. 7A, in the case where the clocked inverter326 is used as a sense amplifier as illustrated in FIG. 9B, the throughcurrent can be further suppressed as compared to the case where theinverter 325 is used as a sense amplifier as illustrated in FIG. 5B.

The data of the memory cell 170 can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling the time of reading with theCLK signal, current consumption in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. When the potential of the CLK signal is H potential fora short time as illustrated in FIG. 9B, current consumption iseffectively reduced. Note that this is based on the premise that thereis no harm in reading operation.

In the case where the memory cell 170 is multi-valued, a resistancevalue of the transistor 160 in the memory 170 is changed step by step.Therefore, the potential of the bit line BL is likely to be anintermediate potential due to the resistance division of the load 323 inthe reading circuit and the transistor 160.

In that case, as illustrated in FIGS. 5A and 5B, as the potential of theinput signal is closer to (VDD/2), the through current is larger in somecases depending on an element used as the sense amplifier illustrated inFIG. 3A. In addition, in the reading operation of the memory cell 170,since a plurality of bits can be read out at one time, a large amount ofcurrent consumption may be caused. In order to prevent the throughcurrent, a circuit configuration including a comparator or the likeinstead of the sense amplifier can be employed; however, the area of thecircuit is increased.

Thus, as illustrated in FIGS. 6A and 6B and FIG. 8, with the use of theclocked inverter 326 instead of the sense amplifier, the through currentcan be reduced. Accordingly, the through current flowing through thereading circuit can be suppressed in the reading operation of the memorycell 170. In addition, even when a plurality of bits are read out at atime, power consumption can be reduced. Further, a memory readingcircuit with a small number of circuit components can be realized. Thesize of the reading circuit can be reduced. Thus, the space of thememory peripheral circuit can be saved and current consumption can bereduced.

Next, other examples of a reading circuit which are different from FIGS.6A and 6B and FIG. 8 are described with reference to FIG. 10 and FIG.12. In each of FIG. 10 and FIG. 12, the clocked inverter 326 controlledby clock signals is used as a sense amplifier. In order to prevent ahigh impedance state of the output of the clocked inverter 326, ap-channel transistor or an n-channel transistor connected to VDD or GNDis connected to the output terminal of the clocked inverter 326. Inaddition, a latch circuit 329 is connected to the output of the clockedinverter 326 and a control signal RE is added, whereby a reading circuitwith higher controllability can be realized.

FIG. 10 illustrates a reading circuit in which the load 323 is connectedto the input of the clocked inverter 326, and the p-channel transistor327 connected to VDD and the latch circuit 329 controlled by the controlsignal RE are connected to the output of the clocked inverter 326. Inaddition, the reading circuit is connected to the memory cell 170through the analog switch 223 a.

FIG. 11 shows a timing chart in reading operation of FIG. 10.

In FIG. 11, in the case where the potential of the capacitor line CL isgradually decreased from H potential to L potential, the potential ofthe floating gate portion FG is gradually decreased from the highpotential side to the low potential side by capacitive coupling in asimilar manner to the capacitor line CL. When the potential of thefloating gate portion FG is decreased from the high potential side tothe low potential side, the resistance value of the transistor 160 ischanged from the high resistance side to the low resistance side. As theresistance value of the transistor 160 is decreased, the potential ofthe bit line BL is raised because the potential of the bit line BL isdetermined by the resistance division of the load 323 in the readingcircuit and the transistor 160.

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate the outputsignal corresponding to the potential of the bit line BL in an inverteroutput signal line INV_OUT whenever the potential of the bit line BL ischanged.

Specifically, in the case where the potential of the CLK signal is Hpotential, since H potential is input to the gate terminal of thetransistor 327 connected to the power supply potential VDD, the powersupply potential VDD and the inverter output signal line INV_OUT arebrought out of conduction, and the inverter output signal line INV_OUTreflects the output of the clocked inverter 326 with respect to thepotential of the bit line BL. When the potential of the bit line BLexceeds a certain potential (e.g., (VDD/2)), the potential output fromthe clocked inverter 326 of the reading circuit to the inverter outputsignal line INV_OUT is changed from H potential to L potential. When thepotential of the CLK signal is L potential, the clocked inverter 326 isin a high impedance state with respect to the inverter output signalline INV_OUT, and VDD and the inverter output signal line INV_OUT arebrought into conduction by the p-channel transistor 327 connected toVDD. Therefore, the inverter output signal line INV_OUT is at Hpotential regardless of the potential of the bit line BL.

Here, the latch circuit 329 generates a potential which is output to theoutput signal line SA_OUT by the control signal RE and the inverteroutput signal line INV_OUT.

Specifically, the latch circuit 329 outputs H potential to the outputsignal line SA_OUT in response to the change of the control signal REfrom L potential to H potential. Here, latch circuit 329 outputs Lpotential to the output signal line SA_OUT in response to the change ofthe inverter output signal line INV_OUT from H potential to L potential.Then, in the case where the output signal line SA_OUT is at L potentialwhile the control signal RE is at H potential, the output signal lineSA_OUT is kept at L potential even when INV_OUT is returned to Hpotential.

As in the description of FIG. 7A, in the case where the clocked inverter326 is used as a sense amplifier as illustrated in FIG. 11, the throughcurrent can be further suppressed as compared to the case where theinverter 325 is used as a sense amplifier as illustrated in FIG. 5B

The p-channel transistor 327 connected to VDD and the latch circuit 329controlled by the control signal RE are connected to the output of theclocked inverter 326, whereby a waveform of the output signal lineSA_OUT can be shaped.

The operation and the timing of the latch circuit 329 are not limited tothe above, and alternatively, a circuit having similar functions may beused.

The data of the memory cell 170 can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling the time of reading with theCLK signal, current consumption in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. When the potential of the CLK signal is H potential fora short time as illustrated in FIG. 11, current consumption iseffectively reduced. Note that this is based on the premise that thereis no harm in reading operation.

FIG. 12 illustrates a reading circuit in which the load 323 is connectedto the input of the clocked inverter 326, and the n-channel transistor328 grounded to GND and the latch circuit 329 controlled by the controlsignal RE are connected to the output of the clocked inverter 326. Inaddition, the reading circuit is connected to the memory cell 170through the analog switch 223 a.

Next, FIG. 13 shows a timing chart in reading operation of FIG. 12. FIG.13 shows the case where the potential of the capacitor line CL isgradually raised from L potential to H potential.

In FIG. 13, in the case where the potential of the capacitor line CL isgradually raised from L potential to H potential, the potential of thefloating gate portion FG is gradually raised from the low potential sideto the high potential side by capacitive coupling in a similar manner tothe capacitor line CL. When the potential of the floating gate portionFG is raised from the low potential side to the high potential side, theresistance value of the transistor 160 is changed from the lowresistance side to the high resistance side. As the resistance value ofthe transistor 160 is raised, the potential of the bit line BL isdecreased because the potential of the bit line BL is determined by theresistance division of the load 323 in the reading circuit and thetransistor 160,

Here, a pulse of H potential is generated in the CLK signal linewhenever the potential of the capacitor line CL is changed. Accordingly,the clocked inverter 326 of the reading circuit can generate the outputsignal corresponding to the potential of the bit line BL in an inverteroutput signal line INV_OUT whenever the potential of the bit line BL ischanged.

Specifically, in the case where the potential of the CLKB signal is Lpotential, since L potential is input to the gate terminal of thetransistor 328 grounded to the ground potential GND, the groundpotential GND and the inverter output signal line INV_OUT are broughtout of conduction, and the inverter output signal line INV_OUT reflectsthe output of the clocked inverter 326 with respect to the potential ofthe bit line BL. When the potential of the bit line BL is less than acertain potential (e.g., (VDD/2)), the potential output from the clockedinverter 326 of the reading circuit to the inverter output signal lineINV_OUT is changed from L potential to H potential. When the potentialof the CLKB signal is H potential, the clocked inverter 326 is in a highimpedance state with respect to the inverter output signal line INV_OUT,and GND and the inverter output signal line INV_OUT are brought intoconduction by the n-channel transistor 328 connected to VDD. Therefore,the inverter output signal line INV_OUT is at L potential regardless ofthe potential of the bit line BL.

Here, the latch circuit 329 generates a potential which is output to theoutput signal line SA_OUT by the control signal RE and the inverteroutput signal line INV_OUT.

Specifically, the latch circuit 329 outputs H potential to the outputsignal line SA_OUT in response to the change of the control signal REfrom L potential to H potential. Here, latch circuit 329 outputs Lpotential to the output signal line SA_OUT in response to the change ofthe inverter output signal line INV_OUT from L potential to H potential.Then, in the case where the output signal line SA_OUT is at L potentialwhile the control signal RE is at H potential, the output signal lineSA_OUT is kept at L potential even when INV_OUT is returned to Lpotential.

As in the description of FIG. 7A, in the case where the clocked inverter326 is used as a sense amplifier as illustrated in FIG. 13, the throughcurrent can be further suppressed as compared to the case where theinverter 325 is used as a sense amplifier as illustrated in FIG. 5B

The p-channel transistor 327 connected to VDD and the latch circuit 329controlled by the control signal RE are connected to the output of theclocked inverter 326, whereby a waveform of the output signal lineSA_OUT can be shaped.

The operation and the timing of the latch circuit 329 are not limited tothe above, and alternatively, a circuit having similar functions may beused.

The data of the memory cell can be determined by the output of theclocked inverter 326 at the time when the potential of the CLK signal isH potential. In addition, by controlling the time of reading with theCLK signal, current consumption in reading operation can be reduced.

Here, the ratio of a time in which the potential of the CLK signal is Hpotential to a time in which the potential thereof is L potential is notnecessarily 1:1. When the potential of the CLK signal is H potential fora short time as illustrated in FIG. 13, current consumption iseffectively reduced. Note that this is based on the premise that thereis no harm in reading operation.

As described above, with the use of the clocked inverter as a specificcircuit example of a sense amplifier, a memory reading circuit in whicha through current is suppressed can be achieved with a small number ofcircuit components. Accordingly, the space of the memory peripherycircuit can be saved and the current consumption can be reduced.Further, the space of the semiconductor device can be saved and thecurrent consumption can be reduced.

Next, an example of a semiconductor device to which any of FIGS. 3A to3C to FIG. 13 can be applied is described.

Specifically, an example of a circuit configuration is described, inwhich eight input-output data signal lines I/O are included, and 4-bitdata (16 values (2⁴ values)) are written and read to/from one memorycell. H potential denotes VDD and L potential denotes GND unlessotherwise specified.

FIG. 14 is an example of a block diagram of a semiconductor device. Thesemiconductor device illustrated in FIG. 14 includes the memory cellarray 201 including a plurality of memory cells 170, the column drivercircuit 202, the row driver circuit 203, a controller 204, a counter206, an I/O control circuit 205, and a potential generating circuit 207.

The memory cell array 201 is connected to the column driver circuit 202that controls the bit line BL and the source line SL, and the row drivercircuit 203 that controls the gate line GL and the capacitor line CL.The column driver circuit 202 is connected to the potential generatingcircuit 207, the counter 206, and the I/O control circuit 205. The rowdriver circuit 203 is connected to the potential generating circuit 207.In addition, the potential generating circuit 207 is connected to thecounter 206. These circuits except the memory cell array 201 areconnected to the controller 204.

The eight input-output data signal lines I/O1 to I/O8 are connected tothe I/O control circuit 205 and connected to the column driver circuit202 through input data signal lines DIN1 to DIN8 and output data signallines DOUT1 to DOUT8. Further, the I/O control circuit 205 is controlledby the controller 204. For example, in the case where H potential isinput to the I/O control circuit 205 through a control line connected tothe controller 204, signals of the eight input-output data signal linesI/O1 to I/O8 are input to the I/O control circuit 205 and the eightinput-output data signal lines I/O1 to I/O8 and the eight input datasignal lines DIN1 to DIN8 are brought into conduction, respectively, andthe signals are output to the column driver circuit 202. In addition, inthe case where L potential is input to the I/O control circuit 205through the control line connected to the controller 204, signals of theeight output data signal lines DOUT1 to DOUT8 are input from the columndriver circuit 202 to the I/O control circuit 205, the eight output datasignal lines DOUT1 to DOUT8 and the eight input-output data signal linesI/O1 to I/O8 are brought into conduction, respectively, and the signalsare output to the input-output data signal lines I/O1 to I/O8.

The counter 206 is connected to the column driver circuit 202 and thepotential generating circuit 207 through counter signal lines COUNT1 toCOUNT4. Further, the counter 206 is controlled by the controller 204 andoutputs 4-bit data of the counter signal lines COUNT1 to COUNT4 to thecolumn driver circuit 202 and the potential generating circuit 207.

The potential generating circuit 207 is connected to the column drivercircuit 202 through analog power supply voltage lines V1 to V16 and aconstant-voltage power supply line Vread and connected to the row drivercircuit 203 through a variable power supply line VR. The potentialgenerating circuit 207 is controlled by the controller 204. Thepotential generating circuit 207 outputs a high power supply voltage VH,the voltages of the analog power supply voltage lines V1 to V16, and thevoltage of the constant-voltage power supply line Vread to the columndriver circuit 202, and outputs the voltage of the variable power supplyline VR, which is changed depending on the data of the counter signallines COUNT1 to COUNT4, and the high power supply voltage VH to the rowdriver circuit 203. In this embodiment, the relation of the voltages ofthe analog power supply voltage lines V1 to V16 isV1<V2<V3<V4<V5<V6<V7<V8<V9<V10<V11<V12<V13<V14<V15<V16<VH. In addition,the voltage of the analog power supply voltage line V1 is GND. As thedata of the counter signal lines COUNT1 to COUNT4 are smaller, thevoltage of the variable power supply line VR is larger. Note that thevariable power supply line VR is controlled by the controller 204. Thevariable power supply line VR outputs a voltage in accordance with thedata of the counter signal lines COUNT1 to COUNT4 at the time of readingoperation and outputs L potential in the other cases.

The above described reading circuit is provided in the bit line/sourceline 221 illustrated in FIG. 14. With the use of the clocked inverter326 in the reading circuit, a memory reading circuit in which a throughcurrent is suppressed can be realized with a small number of circuitcomponents. Accordingly, the space of a memory peripheral circuit can besaved and current consumption can be reduced. Further, the space of asemiconductor device can be saved and current consumption can bereduced.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

[Embodiment 2]

In this embodiment, a structure of a semiconductor device and a methodfor manufacturing the semiconductor device according to one embodimentof the present invention will be described with reference to FIGS. 17Aand 17B, FIGS. 18A to 18G, FIGS. 19A to 19E, FIGS. 20A to 20D, FIGS. 21Ato 21D, and FIGS. 22A to 22C.

<Cross-sectional Structure and Planar Structure of Semiconductor Device>

FIGS. 17A and 17B illustrate an example of a structure of asemiconductor device. FIG. 17A illustrates a cross section of thesemiconductor device, and FIG. 17B illustrates a plan view of thesemiconductor device. Here, FIG. 17A corresponds to the cross sectionalong lines A1-A2 and B1-B2 in FIG. 17B. The semiconductor deviceillustrated in FIGS. 17A and 17B includes a transistor 160 including afirst semiconductor material in a lower portion, and a transistor 162including a second semiconductor material in an upper portion. Here, thefirst semiconductor material is preferably different from the secondsemiconductor material. For example, a semiconductor material except anoxide semiconductor can be used as the first semiconductor material, andan oxide semiconductor can be used as the second semiconductor material.The semiconductor material except an oxide semiconductor can be, forexample, silicon, germanium, silicon germanium, silicon carbide, galliumarsenide, or the like and is preferably single crystalline.Alternatively, an organic semiconductor material or the like may beused. A transistor including such a semiconductor material can operateat high speed easily. On the other hand, a transistor including an oxidesemiconductor can hold charge for a long time owing to itscharacteristics. The semiconductor device in FIGS. 17A and 17B can beused as a memory cell.

Note that the technical feature of one embodiment of the presentinvention is to use a semiconductor material with which off-statecurrent can be sufficiently reduced, such as an oxide semiconductor, inthe transistor 162 in order to hold data. Therefore, it is not necessaryto limit specific conditions such as a material, a structure, and thelike of the semiconductor device to those described here.

The transistor 160 in FIGS. 17A and 17B includes a channel formationregion 134 provided in a semiconductor layer over a semiconductorsubstrate 500, impurity regions 132 (also referred to as a source regionand a drain region) with the channel formation region 134 providedtherebetween, a gate insulating layer 122 a provided over the channelformation region 134, and a gate electrode 128 a provided over the gateinsulating layer 122 a so as to overlap with the channel formationregion 134. Note that a transistor whose source electrode and drainelectrode are not illustrated in a drawing may be referred to as atransistor for the sake of convenience. Further, in such a case, indescription of a connection of a transistor, a source region and asource electrode are collectively referred to as a “source electrode,”and a drain region and a drain electrode are collectively referred to asa “drain electrode”. That is, in this specification, the term “sourceelectrode” may include a source region.

Further, a conductive layer 128 b is connected to an impurity region 126provided in the semiconductor layer over the semiconductor substrate500. Here, the conductive layer 128 b functions as a source electrode ora drain electrode of the transistor 160. In addition, an impurity region130 is provided between the impurity region 132 and the impurity region126. Further, insulating layers 136, 138, and 140 are provided so as tocover the transistor 160. Note that in order to realize higherintegration, the transistor 160 preferably has a structure without asidewall insulating layer as illustrated in FIGS. 17A and 17B. On theother hand, when importance is put on the characteristics of thetransistor 160, sidewall insulating layers may be provided on sidesurfaces of the gate electrode 128 a, and the impurity region 132 mayinclude regions with a different impurity concentrations.

The transistor 162 in FIGS. 17A and 17B includes an oxide semiconductorlayer 144 provided over an insulating layer 140 and the like; a sourceelectrode (or a drain electrode) 142 a and a drain electrode (or asource electrode) 142 b which are electrically connected to the oxidesemiconductor layer 144; a gate insulating layer 146 covering the oxidesemiconductor layer 144, the source electrode 142 a, and the drainelectrode 142 b; and a gate electrode 148 a provided over the gateinsulating layer 146 so as to overlap with the oxide semiconductor layer144.

Here, the oxide semiconductor layer 144 is preferably an oxidesemiconductor layer which is highly purified by sufficiently removingimpurities such as hydrogen or sufficiently supplying oxygen.Specifically, the hydrogen concentration of the oxide semiconductorlayer 144 is 5×10¹⁹ atoms/cm³ or lower, preferably 5×10¹⁸ atoms/cm³ orlower, more preferably 5×10¹⁷ atoms/cm³ or lower. Note that the hydrogenconcentration of the oxide semiconductor layer 144 is measured bysecondary ion mass spectrometry (SIMS). The carrier concentration of theoxide semiconductor layer 144, in which hydrogen is reduced to asufficiently low concentration so that the oxide semiconductor layer ishighly purified and in which defect states in an energy gap due tooxygen deficiency are reduced by sufficiently supplying oxygen, is lowerthan 1×10¹²/cm³, preferably lower than 1×10¹¹/cm³, more preferably lowerthan 1.45×10¹⁰/cm³. For example, the off-state current (per unit channelwidth (1 μm) here) at room temperature (25° C.) is 100 zA (1 zA(zeptoampere) is 1×10⁻²¹ A) or less, preferably 10 zA or less. In thismanner, by using an oxide semiconductor which is made to be an i-type(intrinsic) oxide semiconductor or a substantially i-type oxidesemiconductor, the transistor 162 which has extremely favorableoff-state current characteristics can be obtained.

Note that although the transistor 162 in FIGS. 17A and 17B includes theoxide semiconductor layer 144 which is processed into an island shape inorder to suppress leakage current between elements which is caused dueto miniaturization, the oxide semiconductor layer 144 which is notprocessed into an island shape may be employed. In the case where theoxide semiconductor layer is not processed into an island shape,contamination of the oxide semiconductor layer 144 due to etching in theprocessing can be prevented.

A capacitor 164 in FIGS. 17A and 17B includes the drain electrode 142 b,the gate insulating layer 146, and a conductive layer 148 b. That is tosay, the drain electrode 142 b functions as one of electrodes of thecapacitor 164, and the conductive layer 148 b functions as the other ofthe electrodes of the capacitor 164. With such a structure, capacitancecan be sufficiently secured. Further, insulation between the drainelectrode 142 b and the conductive layer 148 b can be sufficientlysecured by stacking the oxide semiconductor layer 144 and the gateinsulating layer 146. Further alternatively, the capacitor 164 may beomitted in the case where a capacitor is not needed.

In this embodiment, the transistor 162 and the capacitor 164 areprovided so as to overlap with at least part of the transistor 160. Byemploying such a planar layout, high integration can be realized. Forexample, given that the minimum feature size is F, the area occupied bya memory cell can be 15 F² to 25 F².

An insulating layer 150 is provided over the transistor 162 and thecapacitor 164. A wiring 154 is provided in an opening formed in the gateinsulating layer 146 and the insulating layer 150. The wiring 154 is awiring for connecting one memory cell and another memory cell andcorresponds to the bit line BL in FIG. 2. The wiring 154 is connected tothe impurity region 126 through the source electrode 142 a and theconductive layer 128 b. The above structure allows a reduction in thenumber of wirings in comparison with a structure in which the sourceregion or the drain region in the transistor 160 and the sourceelectrode 142 a in the transistor 162 are connected to differentwirings. Thus, the integration degree of a semiconductor device can beincreased.

Since the conductive layer 128 b is provided, a position where theimpurity region 126 and the source electrode 142 a are connected and aposition where the source electrode 142 a and the wiring 154 areconnected can overlap with each other. With such a planar layout, theelement area can be prevented from increasing due to contact regions ofthe electrodes. In other words, the integration degree of thesemiconductor device can be increased.

<Method for Manufacturing SOI Substrate>

Next, an example of a method for manufacturing an SOI substrate used formanufacturing the semiconductor device will be described with referenceto FIGS. 18A to 18G.

First, the semiconductor substrate 500 is prepared as a base substrate(see FIG. 18A). As the semiconductor substrate 500, a semiconductorsubstrate such as a single crystal silicon substrate or a single crystalgermanium substrate can be used. Alternatively, as the semiconductorsubstrate, a solar grade silicon (SOG-Si) substrate or the like may beused. Further alternatively, a polycrystalline semiconductor substratemay be used. In the case of using a SOG-Si substrate, a polycrystallinesemiconductor substrate, or the like, manufacturing cost can be lower ascompared to the case of using a single crystal silicon substrate or thelike.

Note that, in place of the semiconductor substrate 500, a variety ofglass substrates that are used in the electronics industry, such asaluminosilicate glass substrates, aluminoborosilicate glass substrates,and barium borosilicate glass substrates; quartz substrates; ceramicsubstrates; and sapphire substrates can be used. Further, a ceramicsubstrate which contains silicon nitride and aluminum nitride as itsmain components and whose coefficient of thermal expansion is close tothat of silicon may be used.

A surface of the semiconductor substrate 500 is preferably cleaned inadvance. Specifically, the semiconductor substrate 500 is preferablysubjected to cleaning with a hydrochloric acid/hydrogen peroxide mixture(HPM), a sulfuric acid/hydrogen peroxide mixture (SPM), an ammoniumhydrogen peroxide mixture (APM), diluted hydrofluoric acid (DHF), or thelike.

Next, a bond substrate is prepared. Here, a single crystal semiconductorsubstrate 510 is used as the bond substrate (see FIG. 18B). Note thatalthough the substrate whose crystallinity is single crystal is used asthe bond substrate here, the crystallinity of the bond substrate is notnecessarily limited to single crystal.

For example, as the single crystal semiconductor substrate 510, a singlecrystal semiconductor substrate formed using an element of Group 14,such as a single crystal silicon substrate, a single crystal germaniumsubstrate, or a single crystal silicon germanium substrate, can be used.Further, a compound semiconductor substrate using gallium arsenide,indium phosphide, or the like can be used. Typical examples ofcommercially available silicon substrates are circular siliconsubstrates which are 5 inches (125 mm) in diameter, 6 inches (150 mm) indiameter, 8 inches (200 mm) in diameter, 12 inches (300 mm) in diameter,and 16 inches (400 mm) in diameter. Note that the shape of the singlecrystal semiconductor substrate 510 is not limited to circular, and thesingle crystal semiconductor substrate 510 may be a substrate which hasbeen processed into, for example, a rectangular shape or the like.Further, the single crystal semiconductor substrate 510 can be faultedby a Czochralski (CZ) method or a Floating Zone (FZ) method.

An oxide film 512 is formed over a surface of the single crystalsemiconductor substrate 510 (see FIG. 18C). In view of removal ofcontamination, it is preferable that the surface of the single crystalsemiconductor substrate 510 be cleaned with a hydrochloric acid/hydrogenperoxide mixture (RPM), a sulfuric acid/hydrogen peroxide mixture (SPM),an ammonium hydrogen peroxide mixture (APM), diluted hydrofluoric acid(DHF), FPM (a mixed solution of hydrofluoric acid, hydrogen peroxidewater, and pure water), or the like before the formation of the oxidefilm 512. Alternatively, diluted hydrogen fluoride and ozone water maybe discharged alternately for cleaning.

The oxide film 512 can be formed with, for example, a single layer or astacked layer of a silicon oxide film, a silicon oxynitride film, andthe like. As a method for forming the oxide film 512, a thermaloxidation method, a CVD method, a sputtering method, or the like can beused. When the oxide film 512 is formed by a CVD method, a silicon oxidefilm is preferably formed using organosilane such as tetraethoxysilane(abbreviation: TEOS) (chemical formula: Si(OC₂H₅)₄), so that favorablebonding can be achieved.

In this embodiment, the oxide film 512 (here, a SiO_(x) film) is formedby performing thermal oxidation treatment on the single crystalsemiconductor substrate 510. The thermal oxidation treatment ispreferably performed in an oxidizing atmosphere to which a halogen isadded.

For example, thermal oxidation treatment of the single crystalsemiconductor substrate 510 is performed in an oxidation atmosphere towhich chlorine (Cl) is added, whereby the oxide film 512 can be formedthrough chlorine oxidation. In this case, the oxide film 512 is a filmcontaining chlorine atoms. By such chlorine oxidation, heavy metal(e.g., Fe, Cr, Ni, or Mo) that is an extrinsic impurity is trapped andchloride of the metal is formed and then removed to the outside; thus,contamination of the single crystal semiconductor substrate 510 can bereduced.

Note that the halogen atoms contained in the oxide film 512 are notlimited to chlorine atoms. A fluorine atom may be contained in the oxidefilm 512. As a method for fluorine oxidation of the surface of thesingle crystal semiconductor substrate 510, a method in which the singlecrystal semiconductor substrate 510 is soaked in an HF solution and thensubjected to thermal oxidation treatment in an oxidizing atmosphere, amethod in which thermal oxidation treatment is performed in an oxidizingatmosphere to which NF₃ is added, or the like can be used.

Next, ions are accelerated by an electric field and the single crystalsemiconductor substrate 510 is irradiated with the ions and the ions areadded thereto, whereby an embrittled region 514 where the crystalstructure is damaged is formed in the single crystal semiconductorsubstrate 510 at a predetermined depth (see FIG. 18D).

The depth at which the embrittled region 514 is formed can be adjustedby the kinetic energy, mass, charge, or incidence angle of the ions, orthe like. The embrittled region 514 is formed at approximately the samedepth as the average penetration depth of the ions. Therefore, thethickness of the single crystal semiconductor layer to be separated fromthe single crystal semiconductor substrate 510 can be adjusted with thedepth at which the ions are added. For example, the average penetrationdepth may be controlled such that the thickness of a single crystalsemiconductor layer is approximately 10 nm to 500 nm, preferably, 50 nmto 200 nm.

The above ion irradiation treatment can be performed with an ion-dopingapparatus or an ion-implantation apparatus. As a typical example of theion-doping apparatus, there is a non-mass-separation type apparatus inwhich plasma excitation of a process gas is performed and an object isirradiated with all kinds of ion species generated. In this apparatus,the object is irradiated with ion species of plasma without massseparation. In contrast, an ion-implantation apparatus is amass-separation apparatus. In the ion-implantation apparatus, massseparation of ion species of plasma is performed and the object isirradiated with ion species having predetermined masses.

In this embodiment, an example is described in which an ion-dopingapparatus is used to add hydrogen to the single crystal semiconductorsubstrate 510. A gas containing hydrogen is used as a source gas. As forions used for the irradiation, the proportion of H₃+ is preferably sethigh. Specifically, it is preferable that the proportion of H₃ ⁺ be set50% or higher (more preferably, 80% or higher) with respect to the totalamount of H⁺, H₂ ⁺, and H₃ ⁺. With a high proportion of H₃ ⁺, theefficiency of ion irradiation can be improved.

Note that ions to be added are not limited to ions of hydrogen. Ions ofhelium or the like may be added. Further, ions to be added are notlimited to one kind of ions, and plural kinds of ions may be added. Forexample, in the case of performing irradiation with hydrogen and heliumconcurrently using an ion-doping apparatus, the number of steps can besmaller as compared to the case of performing irradiation with hydrogenand helium in different steps, and surface roughness of a single crystalsemiconductor layer to be formed later can be suppressed.

Note that heavy metal may also be added when the embrittled region 514is formed with the ion-doping apparatus; however, the ion irradiation isperformed through the oxide film 512 containing halogen atoms, wherebycontamination of the single crystal semiconductor substrate 510 due tothe heavy metal can be prevented.

Next, the semiconductor substrate 500 and the single crystalsemiconductor substrate 510 are disposed to face each other and thendisposed in close contact with each other with the oxide film 512provided therebetween. Thus, the semiconductor substrate 500 and thesingle crystal semiconductor substrate 510 can be bonded to each other(see FIG. 18E). Note that an oxide film or a nitride film may bedeposited over a surface of the semiconductor substrate 500 bonded tothe single crystal semiconductor substrate 510.

When bonding is performed, it is preferable that a pressure greater thanor equal to 0.001 N/cm² and less than or equal to 100 N/cm², e.g., apressure greater than or equal to 1 N/cm² and less than or equal to 20N/cm², be applied to one part of the semiconductor substrate 500 or onepart of the single crystal semiconductor substrate 510. When the bondingsurfaces are made close to each other and disposed in close contact witheach other by applying a pressure, a bonding between the semiconductorsubstrate 500 and the oxide film 512 is generated at the part where theclose contact is made, and the bonding spontaneously spreads to almostthe entire area. This bonding is performed under the action of the Vander Waals force or hydrogen bonding and can be performed at roomtemperature.

Note that before the single crystal semiconductor substrate 510 and thesemiconductor substrate 500 are bonded to each other, the surfaces to bebonded to are preferably subjected to surface treatment. Surfacetreatment can improve the bonding strength at the interface between thesingle crystal semiconductor substrate 510 and the semiconductorsubstrate 500.

As the surface treatment, wet treatment, dry treatment, or a combinationof wet treatment and dry treatment can be used. Alternatively, wettreatment may be used in combination with different wet treatment or drytreatment may be used in combination with different dry treatment.

Note that heat treatment for increasing the bonding strength may beperformed after bonding. This heat treatment is performed at atemperature at which separation at the embrittled region 514 does notoccur (for example, a temperature higher than or equal to roomtemperature and lower than 400° C.). Alternatively, bonding of thesemiconductor substrate 500 and the oxide film 512 may be performedwhile heating them at a temperature in this range. The heat treatmentcan be performed using a diffusion furnace, a heating furnace such as aresistance heating furnace, a rapid thermal annealing (RTA) apparatus, amicrowave heating apparatus, or the like. Note that the abovetemperature condition is merely an example, and one embodiment of thepresent invention should not be construed as being limited to thisexample.

Next, heat treatment is performed for separation of the single crystalsemiconductor substrate 510 at the embrittlement region, whereby asingle crystal semiconductor layer 516 is formed over the semiconductorsubstrate 500 with the oxide film 512 provided therebetween (FIG. 18F).

Note that the temperature for heat treatment in the separation isdesirably as low as possible. This is because as the temperature in theseparation is low, generation of roughness on the surface of the singlecrystal semiconductor layer 516 can be suppressed. Specifically, thetemperature of the heat treatment in the separation may be higher thanor equal to 300° C. and lower than or equal to 600° C., and the heattreatment is more effective when the temperature is lower than or equalto 500° C. (higher than or equal to 400° C.).

Note that after the single crystal semiconductor substrate 510 isseparated, the single crystal semiconductor layer 516 may be subjectedto heat treatment at 500° C. or higher so that concentration of hydrogenremaining in the single crystal semiconductor layer 516 is reduced.

Then, the surface of the single crystal semiconductor layer 516 isirradiated with laser light, whereby a single crystal semiconductorlayer 518 in which the planarity of the surface is improved and thenumber of defects is reduced is formed (see FIG. 18G). Note that insteadof the laser light irradiation treatment, heat treatment may beperformed.

Although the irradiation treatment with the laser light is performedimmediately after the heat treatment for separation of the singlecrystal semiconductor layer 516 in this embodiment, one embodiment ofthe present invention is not construed as being limited to this. Etchingtreatment may be performed after the heat treatment for separation ofthe single crystal semiconductor layer 516, to remove a region wherethere are many defects on the surface of the single crystalsemiconductor layer 516, and then the laser light irradiation treatmentmay be performed. Alternatively, after the surface planarity of thesingle crystal semiconductor layer 516 is improved, the laser lightirradiation treatment may be performed. Note that the etching treatmentmay be either wet etching or dry etching. Further, in this embodiment, astep of reducing the thickness of the single crystal semiconductor layer516 may be performed after the laser light irradiation. In order toreduce the thickness of the single crystal semiconductor layer 516, anyone or both of dry etching and wet etching may be employed.

Through the above steps, an SOI substrate having the single crystalsemiconductor layer 518 with favorable characteristics can be obtained(see FIG. 18G).

<Method for Manufacturing Semiconductor Device>

Next, a method for manufacturing a semiconductor device in which theabove SOT substrate is used will be described with reference to FIGS.19A to 19E, FIGS. 20A to 20D, FIGS. 21A to 21D, and FIGS. 22A to 22C.

<Method for Manufacturing Transistor in Lower Portion>

First, a method for manufacturing the transistor 160 in a lower portionis described with reference to FIGS. 19A to 19E and FIGS. 20A to 20D.Note that FIGS. 19A to 19E and FIGS. 20A to 20D illustrate part of theSOI substrate formed by the method illustrated in FIGS. 18A to 18G, andare cross-sectional process views illustrating the transistor in thelower portion illustrated in FIG. 17A.

First, the single crystal semiconductor layer 518 is patterned into anisland shape so that a semiconductor layer 120 is formed (see FIG. 19A).Note that before or after this step, an impurity element impartingn-type conductivity or an impurity element imparting p-type conductivitymay be added to the semiconductor layer in order to control thethreshold voltage of the transistor. In the case where silicon is usedas the semiconductor, phosphorus, arsenic, or the like can be used as animpurity element imparting n-type conductivity. On the other hand,boron, aluminum, gallium, or the like can be used as an impurity elementimparting p-type conductivity.

Next, an insulating layer 122 is formed so as to cover the semiconductorlayer 120 (see FIG. 19B). The insulating layer 122 is to be a gateinsulating layer later. The insulating layer 122 can be formed, forexample, by performing heat treatment (thermal oxidation treatment,thermal nitridation treatment, or the like) on a surface of thesemiconductor layer 120. Instead of heat treatment, high-density plasmatreatment may be employed. The high-density plasma treatment can beperformed using, for example, a mixed gas of a rare gas such as He, Ar,Kr, or Xe and any of oxygen, nitrogen oxide, ammonia, nitrogen, andhydrogen. Needless to say, the insulating layer may be formed using aCVD method, a sputtering method, or the like. The insulating layer 122preferably has a single-layer structure or a layered structure using afilm including any of silicon oxide, silicon oxynitride, siliconnitride, hafnium oxide, aluminum oxide, tantalum oxide, yttrium oxide,hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate(HfSi_(x)O_(y) (x>0, y>0)) to which nitrogen is added, hafnium aluminate(HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added, and the like. Thethickness of the insulating layer 122 may be, for example, greater thanor equal to 1 nm and less than or equal to 100 nm, preferably greaterthan or equal to 10 nm and less than or equal to 50 nm. Here, asingle-layer insulating layer containing silicon oxide is formed using aplasma CVD method.

Next, a mask 124 is formed over the insulating layer 122 and an impurityelement imparting one conductivity type is added to the semiconductorlayer 120, so that the impurity region 126 is formed (see FIG. 19C).Note that here, the mask 124 is removed after the impurity element isadded.

Next, a mask is formed over the insulating layer 122 and a region of theinsulating layer 122 that overlaps with the impurity region 126 ispartly removed, so that the gate insulating layer 122 a is formed (seeFIG. 19D). Part of the insulating layer 122 can be removed by etchingtreatment such as wet etching or dry etching.

Next, a conductive layer for forming a gate electrode (including awiring formed using the same layer as the gate electrode) is formed overthe gate insulating layer 122 a and is processed, so that the gateelectrode 128 a and the conductive layer 128 b are formed (see FIG.19E).

The conductive layer used for the gate electrode 128 a and theconductive layer 128 b can be formed using a metal material such asaluminum, copper, titanium, tantalum, or tungsten. The layer including aconductive material may be formed using a semiconductor material such aspolycrystalline silicon. There is no particular limitation on the methodfor forming the layer containing a conductive material, and a variety ofdeposition methods such as an evaporation method, a CVD method, asputtering method, or a spin coating method can be employed. Theconductive layer may be processed by etching using a resist mask.

Next, an impurity element imparting one conductivity type is added tothe semiconductor layer with the use of the gate electrode 128 a and theconductive layer 128 b as masks, so that the channel formation region134, the impurity region 132, and the impurity region 130 are formed(see FIG. 20A). Here, an impurity element such as boron (B) is added inorder to form a p-channel transistor. Here, in the case of forming ann-channel transistor, an impurity element such as phosphorus (P) orarsenic (As) may be added. Here, the concentration of an impurityelement to be added can be set as appropriate. In addition, after theimpurity element is added, heat treatment for activation is performed.Here, the concentration in the impurity region is increased in thefollowing order: the impurity region 126, the impurity region 132, andthe impurity region 130.

Next, the insulating layer 136, the insulating layer 138, and theinsulating layer 140 are formed so as to cover the gate insulating layer122 a, the gate electrode 128 a, and the conductive layer 128 b (seeFIG. 20B).

The insulating layer 136, the insulating layer 138, and the insulatinglayer 140 can be formed using a material including an inorganicinsulating material such as silicon oxide, silicon oxynitride, siliconnitride oxide, silicon nitride, or aluminum oxide. The insulating layer136, the insulating layer 138, and the insulating layer 140 areparticularly preferably formed using a low dielectric constant (low-k)material, because capacitance due to overlapping electrodes or wiringscan be sufficiently reduced. Note that the insulating layer 136, theinsulating layer 138, and the insulating layer 140 may be porousinsulating layers formed using any of these materials. Since the porousinsulating layer has low dielectric constant as compared to a denseinsulating layer, capacitance due to electrodes or wirings can befurther reduced. Alternatively, the insulating layer 136, the insulatinglayer 138, and the insulating layer 140 can be formed using an organicinsulating material such as polyimide or acrylic. In this embodiment,the case of using silicon oxynitride for the insulating layer 136,silicon nitride oxide for the insulating layer 138, and silicon oxidefor the insulating layer 140 will be described. A layered structure ofthe insulating layer 136, the insulating layer 138, and the insulatinglayer 140 is employed here; however, one embodiment of the presentinvention is not limited to this. A single-layer structure, a layeredstructure of two layers, or a layered structure of four or more layersmay also be used.

Next, the insulating layer 138 and the insulating layer 140 aresubjected to chemical mechanical polishing (CMP) treatment or etchingtreatment, so that the insulating layer 138 and the insulating layer 140are planarized (see FIG. 20C). Here, CMP treatment is performed untilthe insulating layer 138 is partly exposed. When silicon nitride oxideis used for the insulating layer 138 and silicon oxide is used for theinsulating layer 140, the insulating layer 138 functions as an etchingstopper.

Next, the insulating layer 138 and the insulating layer 140 aresubjected to CMP treatment or etching treatment, so that upper surfacesof the gate electrode 128 a and the conductive layer 128 b are exposed(see FIG. 20D). Here, etching treatment is performed until the gateelectrode 128 a and the conductive layer 128 b are partly exposed. Forthe etching treatment, dry etching is preferably performed, but wetetching may be performed. In the step of partly exposing the gateelectrode 128 a and the conductive layer 128 b, in order to improve thecharacteristics of the transistor 162 which is formed later, thesurfaces of the insulating layer 136, the insulating layer 138, and theinsulating layer 140 are preferably planarized as much as possible.

Through the above steps, the transistor 160 in the lower portion can beformed (see FIG. 20D).

Note that before or after the above steps, a step for forming anadditional electrode, wiring, semiconductor layer, or insulating layermay be performed. For example, a multilayer wiring structure in which aninsulating layer and a conductive layer are stacked is employed as awiring structure, so that a highly-integrated semiconductor device canbe provided.

<Method for Manufacturing Transistor in Upper Portion>

Next, a method for manufacturing the transistor 162 in the upper portionwill be described with reference to FIGS. 21A to 21D and FIGS. 22A to22C.

First, an oxide semiconductor layer is formed over the gate electrode128 a, the conductive layer 128 b, the insulating layer 136, theinsulating layer 138, the insulating layer 140, and the like and isprocessed, so that the oxide semiconductor layer 144 is formed (see FIG.21A). Note that an insulating layer functioning as a base may be formedover the insulating layer 136, the insulating layer 138, and theinsulating layer 140 before the oxide semiconductor layer is formed. Theinsulating layer can be formed by a PVD method such as a sputteringmethod, or a CVD method such as a plasma CVD method.

An oxide semiconductor to be used preferably contains at least indium(In) or zinc (Zn). In particular, In and Zn are preferably contained. Asa stabilizer for reducing change in electric characteristics of atransistor using the oxide semiconductor, gallium (Ga) is preferablyadditionally contained. Tin (Sn) is preferably contained as astabilizer. Hafnium (Hf) is preferably contained as a stabilizer.Aluminum (Al) is preferably contained as a stabilizer.

As another stabilizer, one or plural kinds of lanthanoid such as lantern(La), cerium (Ce), praseodymium (Pr), neodymium (Nd), samarium (Sm),europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium (Dy), holmium(Ho), erbium (Er), thulium (Tm), ytterbium (Yb), or lutetium (Lu) may becontained.

As a material used for the oxide semiconductor layer, a four-componentmetal oxide material such as an In—Sn—Ga—Zn—O-based material, anIn—Hf—Ga—Zn—O-based material, an In—Al—Ga—Zn—O-based material, anIn—Sn—Al—Zn—O-based material, an In—Sn—Hf—Zn—O-based material, orIn—Hf—Al—Zn—O-based material; a three-component metal oxide materialsuch as an In—Ga—Zn—O-based material, an In—Sn—Zn—O-based material, anIn—Al—Zn—O-based material, a Sn—Ga—Zn—O-based material, anAl—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, anIn—Hf—Zn—O-based material, an In—La—Zn—O-based material, anIn—Ce—Zn—O-based material, an In—Pr—Zn—O-based material, anIn—Nd—Zn—O-based material, an In—Sm—Zn—O-based material, anIn—Eu—Zn—O-based material, an In—Gd—Zn—O-based material, anIn—Tb—Zn—O-based material, an In—Dy—Zn—O-based material, anIn—Ho—Zn—O-based material, an In—Er—Zn—O-based material, anIn—Tm—Zn—O-based material, an In—Yb—Zn—O-based material, or anIn—Lu—Zn—O-based material; or a two-component metal oxide material suchas an In—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; an In—O-basedmaterial; a Sn—O-based material; or a Zn—O-based material; or the likecan be used. In addition, the above materials may contain SiO₂. Here,for example, an In—Ga—Zn—O-based material means an oxide film containingindium (In), gallium (Ga), and zinc (Zn), and there is no particularlimitation on the composition ratio. Further, the In—Ga—Zn—O-based oxidesemiconductor may contain an element other than In, Ga, and Zn.

Alternatively, a material represented by a chemical formula,InMO₃(ZnO)_(m) (m>0 is satisfied) may be used as an oxide semiconductor.Here, M represents one or more metal elements selected from Ga, Fe, Al,Mn, and Co. For example, M may be Ga, Ga and Al, Ga and Mn, Ga and Co,or the like. Still alternatively, a material represented byIn₃SnO₅(ZnO)_(n) (n>0 is satisfied, and n is an integer) may be used asan oxide semiconductor.

For example, an In—Ga—Zn—O-based material with an atomic ratio ofIn:Ga:Zn=1:1:1 (=1/3:1/3:1/3) or In:Ga:Zn=2:2:1 (=2/5:2/5:1/5), or anyof oxides whose composition is in the neighborhood of the abovecompositions can be used. Alternatively, an In—Sn—Zn—O-based materialwith an atomic ratio of In:Sn:Zn=1:1:1 (=1/3:1/3:1/3), In:Sn:Zn=2:1:3(=1/3:1/6:1/2), or In:Sn:Zn=2:1:5 (=1/4:1/8:5/8), or any of oxides whosecomposition is in the neighborhood of the above compositions may beused.

However, one embodiment of the present invention is not limited theretoand a material having suitable composition may be used in accordancewith needed semiconductor characteristics (such as mobility, a thresholdvoltage, and variation). Further, in order to obtain neededsemiconductor characteristics, suitable carrier concentration, impurityconcentration, defect density, atomic ratio of metal elements andoxygen, interatomic bond distance, density, and the like are preferablyemployed.

For example, with an In—Sn—Zn—O-based material, it is relatively easy toobtain a high mobility. However, even with an In—Ga—Zn—O-base material,a mobility can be increased by reducing the defect density in the bulk.

Note that for example, the expression “the composition of an oxideincluding In, Ga, and Zn at the atomic ratio, In:Ga:Zn=a:b:c (a+b+c=1),is in the neighborhood of the composition of an oxide including In, Ga,and Zn at the atomic ratio, In:Ga:Zn=A:B:C (A+B+C=1)” means that a, b,and c satisfy the following relation: (a−A)²+(b−B)²+(c−C)²≦r², and r maybe 0.05, for example. The same applies to other oxides.

The oxide semiconductor may be either single crystal ornon-single-crystal. In the latter case, the oxide semiconductor may beeither amorphous or polycrystal. Further, the oxide semiconductor mayhave either an amorphous structure including a portion havingcrystallinity or a non-amorphous structure.

In an oxide semiconductor in an amorphous state, a planar surface can beobtained with relative ease, so that when a transistor is manufacturedwith the use of the oxide semiconductor, interface scattering can bereduced, and relatively high mobility can be obtained with relativeease.

In an oxide semiconductor having crystallinity, defects in the bulk canbe further reduced and when a surface planarity is improved, mobilityhigher than that of an oxide semiconductor layer in an amorphous statecan be obtained. In order to improve the surface planarity, the oxidesemiconductor is preferably formed over a planar surface. Specifically,the oxide semiconductor may be formed over a surface with the averagesurface roughness (Ra) of less than or equal to 1 nm, preferably lessthan or equal to 0.3 nm, more preferably less than or equal to 0.1 nm.

Note that the Ra in this specification refers to a centerline averageroughness obtained by three-dimensionally expanding a centerline averageroughness defined by JIS B0601 so as to be applied to a plane. The Racan be expressed as an “average value of absolute values of deviationsfrom a reference plane to a designated plane”, and is defined with thefollowing formula.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 1} \rbrack\mspace{585mu}} & \; \\{{Ra} = {\frac{1}{S_{0}}{\int_{x_{2}}^{x_{1}}{\int_{y_{2}}^{y_{1}}{{{{f( {x,y} )} - Z_{0}}}{\mathbb{d}x}{\mathbb{d}y}}}}}} & (1)\end{matrix}$

In the above formula, S₀ represents an area of a plane to be measured (arectangular region which is defined by four points represented bycoordinates (x₁, y₁), (x₁, y₂), (x₂, y₁), and (x₂, y₂)), and Z₀represents an average height of the plane to be measured. Ra can bemeasured using an atomic force microscope (AFM).

The thickness of the oxide semiconductor layer is preferably greaterthan or equal to 3 nm and less than or equal to 30 nm. This is becausethe transistor might possibly be normally on when the oxidesemiconductor layer is too thick (e.g., the thickness is 50 nm or more).

The oxide semiconductor layer is preferably formed by a method in whichimpurities such as hydrogen, water, a hydroxyl group, or hydride do notenter the oxide semiconductor layer. For example, a sputtering methodcan be used.

As an In—Ga—Zn—O-based target, for example, a target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:1 [molar ratio] can be used.Note that it is not necessary to limit the material and the compositionratio of the target to the above. For example, a target having acomposition ratio of In₂O₃:Ga₂O₃:ZnO=1:1:2 [molar ratio] can be used.

As a target of an In—Zn—O-based material, a target with the followingcomposition ratio is used: the composition ratio of In:Zn is 50:1 to 1:2in an atomic ratio (In₂O₃:ZnO=25:1 to 1:4 in a molar ratio), preferably20:1 to 1:1 in an atomic ratio (In₂O₃:ZnO=10:1 to 1:2 in a molar ratio),further preferably 15:1 to 1.5:1 in an atomic ratio (In₂O₃:ZnO=15:2 to3:4 in a molar ratio). For example, when a target used for forming anIn—Zn—O-based oxide semiconductor has a composition ratio ofIn:Zn:O=X:Y:Z in an atomic ratio, Z>(1.5X+Y).

In addition, the In—Sn—Zn—O-based material can also be referred to asITZO, and an oxide target having a composition ratio of In:Sn:Zn=1:2:2,In:Sn:Zn=2:1:3, In:Sn:Zn=1:1:1, In:Sn:Zn=20:45:35, or the like in anatomic ratio is used.

The relative density of the oxide target is higher than or equal to 90%and lower than or equal to 100%, preferably higher than or equal to 95%and lower than or equal to 99.9%. This is because, with the use of themetal oxide target with a high relative density, the deposited oxidesemiconductor layer can be a dense film.

In this embodiment, the oxide semiconductor layer is formed by asputtering method using an In—Ga—Zn—O-based target.

The deposition atmosphere may be a rare gas (typically argon)atmosphere, an oxygen atmosphere, or a mixed atmosphere of a rare gasand oxygen. An atmosphere of a high-purity gas from which an impuritysuch as hydrogen, water, a hydroxyl group, or hydride is removed ispreferable in order to prevent hydrogen, water, a hydroxyl group,hydride, or the like from entering the oxide semiconductor layer.

For example, the oxide semiconductor layer can be formed as follows.

First, the substrate is held in a deposition chamber which is kept underreduced pressure, and then is heated so that the substrate temperaturereaches a temperature higher than 200° C. and lower than or equal to500° C., preferably higher than 300° C. and lower than or equal to 500°C., further preferably higher than or equal to 350° C. and lower than orequal to 450° C.

Then, a high-purity gas in which impurities such as hydrogen, water, ahydroxyl group, or hydride are sufficiently removed is introduced intothe deposition chamber from which remaining moisture is being removed,and the oxide semiconductor layer is formed over the substrate with theuse of the target. To remove moisture remaining in the depositionchamber, an entrapment vacuum pump such as a cryopump, an ion pump, or atitanium sublimation pump is desirably used. Further, an evacuation unitmay be a turbo pump provided with a cold trap. In the deposition chamberwhich is evacuated with the cryopump, for example, impurities such ashydrogen, water, a hydroxyl group, or hydride (preferably, also acompound containing a carbon atom) and the like are removed, whereby theconcentration of impurities such as hydrogen, water, a hydroxyl group,and hydride in the oxide semiconductor layer formed in the depositionchamber can be reduced.

In the case where the substrate temperature is low (for example, 100° C.or lower) during deposition, a substance including a hydrogen atom mayenter the oxide semiconductor; thus, it is preferable that the substratebe heated at a temperature in the above range. When the oxidesemiconductor layer is formed with the substrate heated at thetemperature, the substrate temperature is increased, so that hydrogenbonds are cut by heat and the substance including a hydrogen atom isless likely to be taken into the oxide semiconductor layer. Therefore,the oxide semiconductor layer is formed with the substrate heated at thetemperature, whereby the concentration of impurities such as hydrogen,water, a hydroxyl group, or hydride in the oxide semiconductor layer canbe sufficiently reduced. Moreover, damage due to sputtering can bereduced.

As an example of deposition conditions, the following conditions areemployed: the distance between the substrate and the target is 60 mm;the pressure is 0.4 Pa; the direct-current (DC) power is 0.5 kW; thesubstrate temperature is 400° C.; and the deposition atmosphere is anoxygen atmosphere (the proportion of the oxygen flow rate is 100%). Notethat a pulse direct current power source is preferable because powdersubstances (also referred to as particles or dust) generated indeposition can be reduced and the film thickness can be uniform.

Note that before the oxide semiconductor layer is formed by a sputteringmethod, powdery substances (also referred to as particles or dust)attached on a formation surface of the oxide semiconductor layer arepreferably removed by reverse sputtering in which an argon gas isintroduced and plasma is generated. The reverse sputtering refers to amethod in which a voltage is applied to a substrate side to generateplasma in the vicinity of the substrate to modify a surface. Note thatinstead of argon, a gas of nitrogen, helium, oxygen, or the like may beused.

The oxide semiconductor layer can be processed by being etched after amask having a desired shape is formed over the oxide semiconductorlayer. The mask may be formed by a method such as photolithography or anink-jet method. For the etching of the oxide semiconductor layer, eitherwet etching or dry etching may be employed. It is needless to say thatboth of them may be employed in combination.

After that, heat treatment (first heat treatment) may be performed onthe oxide semiconductor layer 144. The heat treatment eliminatessubstances including hydrogen atoms in the oxide semiconductor layer144; thus, the structure of the oxide semiconductor layer 144 can beimproved and defect level in the energy gap can be reduced. The heattreatment is performed in an inert gas atmosphere at a temperaturehigher than or equal to 250° C. and lower than or equal to 700° C.,preferably higher than or equal to 450° C. and lower than or equal to600° C. or lower than a strain point of the substrate. The inert gasatmosphere is preferably an atmosphere which contains nitrogen or a raregas (e.g., helium, neon, or argon) as its main component and does notcontain water, hydrogen, or the like. For example, the purity ofnitrogen or a rare gas such as helium, neon, or argon introduced into aheat treatment apparatus is higher than or equal to 6 N (99.9999%),preferably higher than or equal to 7 N (99.99999%) (that is, theconcentration of the impurities is lower than or equal to 1 ppm,preferably lower than or equal to 0.1 ppm).

The heat treatment can be performed in such a manner that, for example,an object is introduced into an electric furnace including a resistanceheating element or the like, and heated, in a nitrogen atmosphere at450° C. for one hour. The oxide semiconductor layer 144 is not exposedto the air during the heat treatment so that entry of water and hydrogencan be prevented.

The above heat treatment has an effect of removing hydrogen, water, andthe like and can be referred to as dehydration treatment,dehydrogenation treatment, or the like. The heat treatment can beperformed at the timing, for example, before the oxide semiconductorlayer is processed into an island shape, after the gate insulating filmis formed, or the like. Such dehydration treatment or dehydrogenationtreatment may be conducted once or plural times.

Next, a conductive layer for forming a source electrode and a drainelectrode (including a wiring formed in the same layer as the sourceelectrode and the drain electrode) is formed over the oxidesemiconductor layer 144 and the like and is processed, so that thesource and drain electrodes 142 a and 142 b are formed (see FIG. 21B).

The conductive layer can be formed by a PVD method or a CVD method. As amaterial for the conductive layer, an element selected from aluminum,chromium, copper, tantalum, titanium, molybdenum, and tungsten; an alloycontaining any of these elements as a component; or the like can beused. Further, one or more materials selected from manganese, magnesium,zirconium, beryllium, neodymium, and scandium may be used.

The conductive layer can have a single-layer structure or a layeredstructure including two or more layers. For example, the conductivelayer can have a single-layer structure of a titanium film or a titaniumnitride film, a single-layer structure of an aluminum film containingsilicon, a two-layer structure in which a titanium film is stacked overan aluminum film, a two-layer structure in which a titanium film isstacked over a titanium nitride film, or a three-layer structure inwhich a titanium film, an aluminum film, and a titanium film are stackedin this order. Note that the conductive layer having a single-layerstructure of a titanium film or a titanium nitride film has an advantagein that it can be easily processed into the source electrode 142 a andthe drain electrode 142 b having a tapered shape.

Alternatively, the conductive layer may be formed using conductive metaloxide. As the conductive metal oxide, indium oxide (In₂O₃), tin oxide(SnO₂), zinc oxide (ZnO), an indium oxide-tin oxide alloy (In₂O₃—SnO₂,which may be abbreviated to ITO), an indium oxide-zinc oxide alloy(In₂O₃—ZnO), or any of these metal oxide materials in which silicon orsilicon oxide is included can be used.

The conductive layer is preferably etched so that the source electrode142 a and the drain electrode 142 b are formed to have tapered endportions. Here, a taper angle is, for example, preferably greater thanor equal to 30° and less than or equal to 60°. The etching is performedso that the end portions of the source electrode 142 a and the drainelectrode 142 b are tapered, whereby coverage with the gate insulatinglayer 146 formed later can be improved and disconnection can beprevented.

The channel length (L) of the transistor in the upper portion isdetermined by a distance between lower edge portions of the sourceelectrode 142 a and the drain electrode 142 b. Note that for lightexposure for forming a mask used in the case where a transistor with achannel length (L) less than 25 nm is fanned, it is preferable to useextreme ultraviolet rays whose wavelength is as short as severalnanometers to several tens of nanometers. In the light exposure byextreme ultraviolet light, the resolution is high and the focus depth islarge. For these reasons, the channel length (L) of the transistor to beformed later can be in the range of greater than or equal to 10 nm andless than or equal to 1000 nm (1 μm), and the circuit can operate athigher speed. Moreover, miniaturization can lead to low powerconsumption of a semiconductor device.

As another example which is different from FIG. 21B, oxide conductivelayers may be provided as a source region and a drain region between theoxide semiconductor layer 144 and the source electrode and between theoxide semiconductor layer 144 and the drain electrode.

For example, an oxide conductive film is formed over the oxidesemiconductor layer 144, a conductive layer is formed thereover, and theoxide conductive film and the conductive layer are processed through thesame photolithography step, so that the oxide conductive layers servingas the source region and the drain region, the source electrode 142 a,and the drain electrode 142 b can be formed.

Alternatively, a stack of an oxide semiconductor film and an oxideconductive film is formed and the shape of the stack of the oxidesemiconductor film and the oxide conductive film is processed throughthe same photolithography step, so that the oxide semiconductor layer144 and an oxide semiconductor film which have island shapes are formed.After the source electrode 142 a and the drain electrode 142 b areformed, the island-shaped oxide conductive film is further etched usingthe source electrode 142 a and the drain electrode 142 b as masks, sothat the oxide conductive layers serving as the source region and thedrain region can be formed.

Note that in the etching treatment for processing the shape of the oxideconductive layers, etching conditions (the kind of etchant, theconcentration, the etching time, and the like) are appropriatelyadjusted so that the oxide semiconductor layer is not excessivelyetched.

A material of the oxide conductive layers preferably contains zinc oxideas a component and preferably does not contain indium oxide. For suchoxide conductive layers, zinc oxide, zinc aluminum oxide, zinc aluminumoxynitride, zinc gallium oxide, or the like can be used.

When the oxide conductive layers are provided between the oxidesemiconductor layer and the source and drain electrodes, the sourceregion and the drain region can have lower resistance and the transistorcan operate at high speed.

With the structure of the oxide semiconductor layer 144, the oxideconductive layers, and the source and drain electrodes formed using ametal material, withstand voltage of the transistor can be furtherincreased.

It is effective to use the oxide conductive layers for the source regionand the drain region in order to improve frequency characteristics of aperipheral circuit (a driver circuit). This is because the contact of ametal electrode (e.g., molybdenum or tungsten) with the oxidesemiconductor layer can further reduce contact resistance than thecontact of a metal electrode (e.g., molybdenum or tungsten) with theoxide conductive layer. The contact resistance can be reduced byinterposing the oxide conductive layers between the oxide semiconductorlayer and the source and drain electrodes; accordingly, frequencycharacteristics of a peripheral circuit (a driver circuit) can beimproved.

Next, the gate insulating layer 146 is formed so as to cover the sourceelectrode 142 a and the drain electrode 142 b and to be in contact withpart of the oxide semiconductor layer 144 (see FIG. 21C).

The gate insulating layer 146 can be formed by a CVD method, asputtering method, or the like. In addition, the gate insulating layer146 is preferably formed so as to contain silicon oxide, siliconnitride, silicon oxynitride, gallium oxide, aluminum oxide, tantalumoxide, hafnium oxide, yttrium oxide, hafnium silicate (HfSi_(x)O_(y)(x>0, y >0)), hafnium silicate (HfSi_(x)O_(y) (x>0, y>0)) to whichnitrogen is added, hafnium aluminate (HfAl_(x)O_(y) (x>0, y>0)) to whichnitrogen is added, or the like. The gate insulating layer 146 may have asingle-layer structure or a layered structure including a combination ofthe above materials. There is no particular limitation on the thickness;however, in the case where a semiconductor device is miniaturized, thethickness is preferably small for ensuring operation of the transistor.For example, in the case where silicon oxide is used, the thickness canbe set to greater than or equal to 1 nm and less than or equal to 100nm, preferably greater than or equal to 10 nm and less than or equal to50 nm.

When the gate insulating layer is thin as described above, a problem ofgate leakage due to a tunnel effect or the like is caused. In order tosolve the problem of gate leakage, it is preferable that the gateinsulating layer 146 be formed using a high dielectric constant (high-k)material such as hafnium oxide, tantalum oxide, yttrium oxide, hafniumsilicate (HfSi_(x)O_(y) (x>0, y>0)), hafnium silicate (HfSi_(x)O_(y)(x>0, y>0)) to which nitrogen is added, or hafnium aluminate(HfAl_(x)O_(y) (x>0, y>0)) to which nitrogen is added. By using a high-kmaterial for the gate insulating layer 146, electrical characteristicscan be ensured and the thickness can be large to prevent gate leakage.Note that a layered structure of a film containing a high-k material anda film containing any one of silicon oxide, silicon nitride, siliconoxynitride, silicon nitride oxide, aluminum oxide, and the like may beemployed.

In addition, an insulating layer (the gate insulating layer 146 in thisembodiment) in contact with the oxide semiconductor layer 144 may beformed using an insulating material containing an element of Group 13and oxygen. Many of oxide semiconductor materials include elements ofGroup 13, and an insulating material containing an element of Group 13is compatible with an oxide semiconductor. Thus, when an insulatingmaterial containing an element of Group 13 is used for an insulatinglayer in contact with the oxide semiconductor layer, the state of theinterface with the oxide semiconductor layer can be kept well.

Here, an insulating material including an element of Group 13 refers toan insulating material including one or more elements of Group 13. Asthe insulating material containing an element of Group 13, a galliumoxide, an aluminum oxide, an aluminum gallium oxide, a gallium aluminumoxide, and the like are given. Here, aluminum gallium oxide refers to amaterial in which the amount of aluminum is larger than that of galliumin atomic percent, and gallium aluminum oxide refers to a material inwhich the amount of gallium is larger than or equal to that of aluminumin atomic percent.

For example, in the case of forming a gate insulating layer in contactwith an oxide semiconductor layer containing gallium, a materialcontaining gallium oxide may be used for the gate insulating layer, sothat favorable characteristics can be kept at the interface between theoxide semiconductor layer and gate the insulating layer. In addition,when the oxide semiconductor layer and the insulating layer containing agallium oxide are provided in contact with each other, pileup ofhydrogen at the interface between the oxide semiconductor layer and theinsulating layer can be reduced. Note that a similar effect can beobtained in the case where an element belonging to the same group as aconstituent element of the oxide semiconductor is used for theinsulating layer. For example, it is effective to form an insulatinglayer with the use of a material containing an aluminum oxide. Aluminumoxide has a property of not easily transmitting water. Thus, it ispreferable to use the material including aluminum oxide in terms ofpreventing entry of water to the oxide semiconductor layer.

An insulating material of the insulating layer in contact with the oxidesemiconductor layer 144 preferably contains oxygen at a proportionhigher than that in the stoichiometric composition, by heat treatment inan oxygen atmosphere, oxygen doping, or the like. “Oxygen doping” refersto addition of oxygen into a bulk. Note that the term “bulk” is used inorder to clarify that oxygen is added not only to a surface of a thinfilm but also to the inside of the thin film. In addition, “oxygendoping” includes “oxygen plasma doping” in which oxygen which is made tobe plasma is added to a bulk. The oxygen doping may be performed usingan ion implantation method or an ion doping method.

For example, in the case where the insulating layer in contact with theoxide semiconductor layer 144 is formed using gallium oxide, thecomposition of gallium oxide can be set to be Ga₂O_(X) (X=3+a, 0<a<1) byheat treatment in an oxygen atmosphere or oxygen doping. In the casewhere the insulating layer in contact with the oxide semiconductor layer144 is formed using aluminum oxide, the composition of aluminum oxidecan be set to be Al₂O_(X) (X=3+a, 0<a<1) by heat treatment in an oxygenatmosphere or oxygen doping. In the case where the insulating layer incontact with the oxide semiconductor layer 144 is formed using galliumaluminum oxide (or aluminum gallium oxide), the composition of galliumaluminum oxide (or aluminum gallium oxide) can be set to beGa_(X)Al_(2-X)O_(3+a) (0<X<2, 0<a<1) by heat treatment in an oxygenatmosphere or oxygen doping.

By oxygen doping treatment or the like, an insulating layer including aregion where the proportion of oxygen is higher than that in thestoichiometric composition can be formed. When the insulating layerincluding such a region is in contact with the oxide semiconductorlayer, oxygen that exists excessively in the insulating layer issupplied to the oxide semiconductor layer, and oxygen deficiency in theoxide semiconductor layer or at an interface between the oxidesemiconductor layer and the insulating layer can be reduced.

Note that instead of the gate insulating layer 146, the insulating layerincluding the region where the proportion of oxygen is higher than thatin the stoichiometric composition may be used for an insulating layerserving as a base film of the oxide semiconductor layer 144 or may beused for both the gate insulating layer 146 and the base insulatinglayer.

After the gate insulating layer 146 is formed, second heat treatment isdesirably performed in an inert gas atmosphere or an oxygen atmosphere.The temperature of the heat treatment is set to higher than or equal to200° C. and lower than or equal to 450° C., preferably higher than orequal to 250° C. and lower than or equal to 350° C. For example, theheat treatment may be performed at 250° C. for one hour in a nitrogenatmosphere. The second heat treatment can reduce variation in electriccharacteristics of the transistor. Further, in the case where the gateinsulating layer 146 contains oxygen, oxygen can be supplied to theoxide semiconductor layer 144 to cover oxygen deficiency in the oxidesemiconductor layer 144.

Note that in this embodiment, the second heat treatment is performedafter the gate insulating layer 146 is formed; however, the timing ofthe second heat treatment is not limited thereto. For example, thesecond heat treatment may be performed after the gate electrode isformed. Alternatively, the second heat treatment may be performedfollowing the first heat treatment, the first heat treatment may doubleas the second heat treatment, or the second heat treatment may double asthe first heat treatment.

By performing at least one of the first heat treatment and the secondheat treatment as described above, the oxide semiconductor layer 144 canbe highly purified so as to include the substance including a hydrogenatom as few as possible.

Next, a conductive layer for forming a gate electrode (including awiring formed using the same layer as the gate electrode) is formed andis processed, so that the gate electrode 148 a and the conductive layer148 b are formed (see FIG. 21D).

The gate electrode 148 a and the conductive layer 148 b can be formedusing a metal material such as molybdenum, titanium, tantalum, tungsten,aluminum, copper, neodymium, or scandium, or an alloy materialcontaining any of these materials as a main component. Note that thegate electrode 148 a and the conductive layer 148 b may have asingle-layer structure or a layered structure.

Next, the insulating layer 150 is formed over the gate insulating layer146, the gate electrode 148 a, and the conductive layer 148 b (see FIG.22A). The insulating layer 150 can be foamed by a PVD method, a CVDmethod, or the like. The insulating layer 150 can be formed using amaterial including an inorganic insulating material such as siliconoxide, silicon oxynitride, silicon nitride, hafnium oxide, galliumoxide, or aluminum oxide. Note that for the insulating layer 150, amaterial with a low dielectric constant may be preferably used or astructure with a low dielectric constant (e.g., a porous structure) maybe preferably employed. This is because by reducing the dielectricconstant of the insulating layer 150, capacitance between wirings andelectrodes can be reduced, which will increase operation speed. Notethat although the insulating layer 150 has a single-layer structure inthis embodiment, one embodiment of the present invention is not limitedto this structure. The insulating layer 150 may have a layered structureincluding two or more layers.

Next, an opening reaching the source electrode 142 a is formed in thegate insulating layer 146 and the insulating layer 150. Then, the wiring154 connected to the source electrode 142 a is formed over theinsulating layer 150 (see FIG. 22B). The opening is formed by selectiveetching using a mask or the like.

A conductive layer is formed by a PVD method or a CVD method and then ispatterned, so that the wiring 154 is formed. As a material for theconductive layer, an element selected from aluminum, chromium, copper,tantalum, titanium, molybdenum, and tungsten; an alloy containing any ofthese elements as a component; or the like can be used. Further, one ormore materials selected from manganese, magnesium, zirconium, beryllium,neodymium, and scandium may be used.

Specifically, it is possible to employ a method, for example, in which athin titanium film (about 5 nm) is formed in a region including theopening of the insulating layer 150 by a PVD method, and then, analuminum film is formed so as to be embedded in the openings. Here, thetitanium film formed by a PVD method functions to reduce an oxide film(e.g., a natural oxide film) formed on a surface where the titanium filmis formed, and to decrease the contact resistance with a lower electrodeor the like (here, the source electrode 142 a). In addition, hillock ofthe aluminum film can be prevented. A copper film may be formed by aplating method after the formation of the barrier film of titanium,titanium nitride, or the like.

The opening formed in the insulating layer 150 is preferably formed in aregion overlapping with the conductive layer 128 b. With the opening insuch a region, the element area can be prevented from increasing due tocontact regions of the electrodes

Here, the case where a connection position of the impurity region 126and the source electrode 142 a and a connection position of the sourceelectrode 142 a and the wiring 154 overlap with each other without usingthe conductive layer 128 b will be described. In this case, an opening(also referred to as a contact in a lower portion) is formed in theinsulating layer 136, the insulating layer 138, and the insulating layer140 that are formed over the impurity region 126, and the sourceelectrode 142 a is formed in the contact in the lower portion. Afterthat, an opening (also referred to as a contact in an upper portion) isformed in a region overlapping with the contact in the lower portion inthe gate insulating layer 146 and the insulating layer 150, and then thewiring 154 is formed. When the contact in the upper portion is formed inthe region overlapping with the contact in the lower portion, the sourceelectrode 142 a formed in the contact in the lower portion by etchingmight be disconnected. In order to avoid the disconnection, the contactsin the lower portion and in the upper portion are formed so as not tooverlap with each other, so that a problem of the increase in theelement area occurs.

As described in this embodiment, with the use of the conductive layer128 b, the contact in the upper portion can be formed withoutdisconnection of the source electrode 142 a. Thus, the contact in thelower portion and in the upper portion can be formed overlapping witheach other, so that the element area can be prevented from increasingdue to contact regions of the electrodes. In other words, theintegration degree of the semiconductor device can be increased.

Next, an insulating layer 156 is formed so as to cover the wiring 154(see FIG. 22C).

Through the above steps, the transistor 162 and the capacitor 164including the highly purified oxide semiconductor layer 144 arecompleted (see FIG. 22C).

Next, an example of a transistor which can be used as the transistor 162illustrated in FIGS. 17A and 17B is described.

Oxide conductive layers serving as a source region and a drain regionmay be provided as buffer layers between the oxide semiconductor layer144 and the source electrode 142 a and between the oxide semiconductorlayer 144 and the drain electrode 142 b. Transistors 441 and 442 eachhaving the structure of the transistor 162 illustrated in FIGS. 17A and17B, in which oxide conductive layers are provided are illustrated inFIGS. 24A and 24B. Note that an insulating layer 400 corresponds to theinsulating layer 136, the insulating layer 138, the insulating layer140, or the like.

In each of the transistors 441 and 442 in FIGS. 24A and 24B, oxideconductive layers 404 a and 404 b serving as a source region and a drainregion are provided between the oxide semiconductor layer 144 and thesource electrode 142 a and between the oxide semiconductor layer 144 andthe drain electrode 142 b. The shapes of the oxide conductive layers 404a and 404 b are different between the transistors 441 and 442 of FIGS.24A and 24B because of the difference between their manufacturingprocesses.

As for the transistor 441 of FIG. 24A, a stack of an oxide semiconductorfilm and an oxide conductive film is formed and the shape of the stackis processed to form the island-shaped oxide semiconductor layer 144 andthe island-shaped oxide conductive film through the samephotolithography step. The source electrode 142 a and the drainelectrode 142 b are formed over the oxide semiconductor layer and theoxide conductive film. After that, the island-shaped oxide conductivefilm is etched with the use of the source electrode 142 a and the drainelectrode 142 b as masks to form the oxide semiconductor conductivelayers 404 a and 404 b serving as the source region and the drainregion.

As for the transistor 442 of FIG. 24B, an oxide conductive film isformed over the oxide semiconductor layer 144, and a metal conductivefilm is formed thereover. Then, the oxide conductive film and the metalconductive film are processed through the same photolithography step toform the oxide conductive layers 404 a and 404 b serving as the sourceregion and the drain region, the source electrode 142 a, and the drainelectrode 142 b.

Note that in the etching treatment for processing the shape of the oxideconductive layer, etching conditions (such as the kind of an etchant,the concentration, or the etching time) are adjusted as appropriate sothat the oxide semiconductor layer is not excessively etched.

As the formation method of the oxide conductive layers 404 a and 404 b,a sputtering method, a vacuum evaporation method (an electron beamevaporation method or the like), an arc discharge ion plating method, ora spray method is used. As a material of the oxide conductive layers,zinc oxide, zinc aluminum oxide, zinc aluminum oxynitride, zinc galliumoxide, indium tin oxide containing silicon oxide, or the like can beused. In addition, the above materials may contain silicon oxide.

When the oxide conductive layers are provided as the source region andthe drain region between the oxide semiconductor layer 144 and thesource electrode 142 a and between the oxide semiconductor layer 144 andthe drain electrode 142 b, the source region and the drain region canhave lower resistance and the transistors 441 and 442 can operate athigh speed.

With the structure including the oxide semiconductor layer 144, theoxide conductive layers 404 a and 404 b, the source electrode 142 a, andthe drain electrode 142 b, withstand voltages of the transistors 441 and442 can be improved.

A top-gate structure is employed as the structure of the transistor 162illustrated in FIGS. 17A and 17B; however, one embodiment of the presentinvention is not limited thereto, and a bottom gate structure may beemployed. FIGS. 26A to 26C illustrate examples of a bottom-gatestructure.

In a transistor 410 illustrated in FIG. 26A, a gate insulating layer 402is provided over the gate electrode 401, an oxide semiconductor layer403 is provided over the gate insulating layer 402, and a sourceelectrode 405 a and a drain electrode 405 b which are connected to theoxide semiconductor layer 403 are provided. Note that the gate electrode401, the oxide semiconductor layer 403, the gate insulating layer 402,the source electrode 405 a, and the drain electrode 405 b correspond tothe gate electrode 148 a, the oxide semiconductor layer 144, the gateinsulating layer 146, the source electrode 142 a, and the drainelectrode 142 b in FIGS. 17A and 17B, respectively.

A transistor 420 illustrated in FIG. 26B are the same as the transistorof FIG. 26A in that the gate electrode 401, the gate insulating layer402, the oxide semiconductor layer 403, the source electrode 405 a, andthe drain electrode 405 b are provided. The transistor of FIG. 26B isdifferent from the transistor of FIG. 26A in that an insulating layer427 is provided in contact with the oxide semiconductor layer 403.

A transistor 430 illustrated in FIG. 26C is the same as the transistorof FIG. 26A in that the gate electrode 401, the gate insulating layer402, the oxide semiconductor layer 403, the source electrode 405 a, andthe drain electrode 405 b are provided. A different point of thetransistor of FIG. 26C from the transistor of FIG. 26A is positionswhere the source electrode 405 a and the drain electrode 405 b are incontact with the oxide semiconductor layer 403. In other words, thesource electrode 405 a and the drain electrode 405 b are provided overand in contact with the oxide semiconductor layer 403 in the transistor410 illustrated in FIG. 26A, whereas the source electrode 405 a and thedrain electrode 405 b are provided below and in contact with the oxidesemiconductor layer 403 in the transistor 430 illustrated in FIG. 26C.

Since the oxide semiconductor layer 144 is highly purified in thetransistor 162 described in this embodiment, the hydrogen concentrationis 5×10¹⁹ atoms/cm³ or lower, preferably 5×10¹⁸ atoms/cm³ or lower, morepreferably 5×10¹⁷ atoms/cm³ or lower. In addition, since oxygendeficiency is reduced and hydrogen, moisture, and the like in the oxidesemiconductor layer 144 is also reduced, the value of the carrierdensity of the oxide semiconductor layer 144 is sufficiently small(e.g., lower than 1×10¹²/cm³, preferably lower than 1.45×10¹⁰/cm³) ascompared with the carrier density of a general silicon wafer(approximately 1×10¹⁴/cm³). Accordingly, the off-state current of thetransistor 162 is also sufficiently small. For example, the off-statecurrent (per unit channel width (1 μm) here) of the transistor 162 atroom temperature (25° C.) is 100 zA (1 zA (zeptoampere) is 1×10⁻²¹ A) orlower, preferably lower than or equal to 10 zA.

By using the oxide semiconductor layer 144 which is highly purified inthis manner, it becomes easy to sufficiently reduce the off-statecurrent of the transistor. Then, by using such a transistor, asemiconductor device in which stored data can be held for an extremelylong time can be obtained.

The structures and methods described in this embodiment can be combinedas appropriate with any of the structures and methods described in theother embodiments.

[Embodiment 3]

One embodiment of an oxide semiconductor layer which can be used as anyof the semiconductor layers of the transistors in the above embodimentswill be described with reference to FIGS. 25A to 25C.

The oxide semiconductor layer of this embodiment has a structureincluding a first crystalline oxide semiconductor layer and a secondcrystalline oxide semiconductor layer which is stacked over the firstcrystalline oxide semiconductor layer and has a larger thickness thanthe first crystalline oxide semiconductor layer.

An insulating layer 437 is formed over an insulating layer 400. In thisembodiment, an oxide insulating layer with a thickness greater than orequal to 50 nm and less than or equal to 600 nm is formed as theinsulating layer 437 by a PCVD method or a sputtering method. Forexample, a single layer selected from a silicon oxide film, a galliumoxide film, an aluminum oxide film, a silicon oxynitride film, analuminum oxynitride film, and a silicon nitride oxide film or a stack ofany of these films can be used. Note that the insulating layer 400corresponds to the insulating layer 136, the insulating layer 138, theinsulating layer 140, and the like.

Next, a first oxide semiconductor film with a thickness greater than orequal to 1 nm and less than or equal to 10 nm is formed over theinsulating layer 437. The first oxide semiconductor film is formed by asputtering method, and the substrate temperature in the deposition by asputtering method is set to be higher than or equal to 200° C. and lowerthan or equal to 400° C.

In this embodiment, the first oxide semiconductor film is deposited to athickness of 5 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where a targetfor an oxide semiconductor (a target for an In—Ga—Zn—O-based oxidesemiconductor including In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]) isused, the distance between the substrate and the target is 170 mm, thesubstrate temperature is 250° C., the pressure is 0.4 Pa, and the directcurrent (DC) power is 0.5 kW.

Next, first heat treatment is performed under a condition where theatmosphere of a chamber in which the substrate is set is an atmosphereof nitrogen or dry air. The temperature of the first heat treatment ishigher than or equal to 400° C. and lower than or equal to 750° C.Through the first heat treatment, a first crystalline oxidesemiconductor layer 450 a is formed (see FIG. 25A).

Depending on the substrate temperature at the time of deposition or thetemperature of the first heat treatment, the first heat treatment causescrystallization from a film surface and crystal grows from the filmsurface toward the inside of the film; thus, c-axis aligned crystal isobtained. By the first heat treatment, large amounts of zinc and oxygengather to the film surface, and one or more layers of graphene-typetwo-dimensional crystal including zinc and oxygen and having a hexagonalupper plane are formed at the outermost surface; the layer(s) at theoutermost surface grow in the thickness direction to form a stack oflayers. By increasing the temperature of the heat treatment, crystalgrowth proceeds from the surface to the inside and further from theinside to the bottom.

By the first heat treatment, oxygen in the insulating layer 437 that isan oxide insulating layer is diffused to an interface between theinsulating layer 437 and the first crystalline oxide semiconductor layer450 a or the vicinity of the interface (within ±5 nm from theinterface), whereby oxygen deficiency in the first crystalline oxidesemiconductor layer is reduced. Therefore, it is preferable that oxygenbe included in (in a bulk of) the insulating layer 437 used as a baseinsulating layer or at the interface between the first crystalline oxidesemiconductor layer 450 a and the insulating layer 437 at an amount thatexceeds at least the amount of oxygen in the stoichiometric compositionratio.

Next, a second oxide semiconductor film with a thickness greater than 10nm is formed over the first crystalline oxide semiconductor layer 450 a.The second oxide semiconductor film is formed by a sputtering method,and the substrate temperature in the deposition is set to be higher thanor equal to 200° C. and lower than or equal to 400° C. By setting thesubstrate temperature in the deposition to be higher than or equal to200° C. and lower than or equal to 400° C., precursors can be arrangedin the oxide semiconductor layer formed over and in contact with thesurface of the first crystalline oxide semiconductor layer and so-calledorderliness can be obtained.

In this embodiment, the second oxide semiconductor film is deposited toa thickness of 25 nm in an oxygen atmosphere, an argon atmosphere, or anatmosphere including argon and oxygen under conditions where a targetfor an oxide semiconductor (a target for an In—Ga—Zn—O-based oxidesemiconductor including In₂O₃, Ga₂O₃, and ZnO at 1:1:2 [molar ratio]) isused, the distance between the substrate and the target is 170 mm, thesubstrate temperature is 400° C., the pressure is 0.4 Pa, and the directcurrent (DC) power is 0.5 kW.

Next, second heat treatment is performed under a condition where theatmosphere of a chamber in which the substrate is set is a nitrogenatmosphere, an oxygen atmosphere, or a mixed atmosphere of nitrogen andoxygen. The temperature of the second heat treatment is higher than orequal to 400° C. and lower than or equal to 750° C. Through the secondheat treatment, a second crystalline oxide semiconductor layer 450 b isformed (see FIG. 25B). The second heat treatment is performed in anitrogen atmosphere, an oxygen atmosphere, or a mixed atmosphere ofnitrogen and oxygen, whereby the density of the second crystalline oxidesemiconductor layer is increased and the number of defects therein isreduced. By the second heat treatment, crystal growth proceeds in thethickness direction with the use of the first crystalline oxidesemiconductor layer 450 a as a nucleus, that is, crystal growth proceedsfrom the bottom to the inside; thus, the second crystalline oxidesemiconductor layer 450 b is formed.

It is preferable that steps from the formation of the insulating layer437 to the second heat treatment be successively performed withoutexposure to the air. The steps from the formation of the insulatinglayer 437 to the second heat treatment are preferably performed in anatmosphere which is controlled to include little hydrogen and moisture(such as an inert gas atmosphere, a reduced-pressure atmosphere, or adry-air atmosphere); in terms of moisture, for example, a dry nitrogenatmosphere with a dew point of −40° C. or lower, preferably a dew pointof −50° C. or lower may be employed.

Next, the stack of the oxide semiconductor layers, the first crystallineoxide semiconductor layer 450 a and the second crystalline oxidesemiconductor layer 450 b, is processed into an oxide semiconductorlayer 453 including a stack of island-shaped oxide semiconductor layers(see FIG. 25C). In the drawing, the interface between the firstcrystalline oxide semiconductor layer 450 a and the second crystallineoxide semiconductor layer 450 b is indicated by a dotted line, and thefirst crystalline oxide semiconductor layer 450 a and the secondcrystalline oxide semiconductor layer 450 b are illustrated as a stackof oxide semiconductor layers; however, the interface is actually notdistinct and is illustrated for easy understanding.

The stack of the oxide semiconductor layers can be processed by beingetched after a mask having a desired shape is formed over the stack ofthe oxide semiconductor layers. The mask can be formed by a method suchas photolithography. Alternatively, the mask may be formed by a methodsuch as an ink-jet method.

For the etching of the stack of the oxide semiconductor layers, eitherdry etching or wet etching may be employed. Needless to say, both ofthem may be employed in combination.

A feature of the first crystalline oxide semiconductor layer and thesecond crystalline oxide semiconductor layer obtained by the aboveformation method is that they have c-axis alignment. Note that the firstcrystalline oxide semiconductor layer and the second crystalline oxidesemiconductor layer comprise an oxide including a crystal with c-axisalignment (also referred to as C-Axis Aligned Crystal (CAAC)), which hasneither a single crystal structure nor an amorphous structure. The firstcrystalline oxide semiconductor layer and the second crystalline oxidesemiconductor layer partly include a crystal grain boundary.

Note that examples of a material for the first crystalline oxidesemiconductor layer and the second crystalline oxide semiconductor layerinclude a four-component metal oxide such as an In—Sn—Ga—Zn—O-basedmaterial; three-component metal oxides such as an In—Ga—Zn—O-basedmaterial (also referred to as IGZO), an In—Sn—Zn—O-based material (alsoreferred to as ITZO), an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-basedmaterial, an Al—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, anIn—Hf—Zn—O-based material, an In—La—Zn—O-based material, anIn—Ce—Zn—O-based material, an In—Pr—Zn—O-based material, anIn—Nd—Zn—O-based material, an In—Sm—Zn—O-based material, anIn—Eu—Zn—O-based material, an In—Gd—Zn—O-based material, anIn—Tb—Zn—O-based material, an In—Dy—Zn—O-based material, anIn—Ho—Zn—O-based material, an In—Er—Zn—O-based material, anIn—Tm—Zn—O-based material, an In—Yb—Zn—O-based material, and anIn—Lu—Zn—O-based material; two-component metal oxides such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, and an In—Ga—O-based material; andsingle-component metal oxides such as an In—O-based material, aSn—O-based material, and a Zn—O-based material. In addition, the abovematerials may contain SiO₂. Here, for example, an In—Ga—Zn—O-basedmaterial means an oxide film containing indium (In), gallium (Ga), andzinc (Zn), and there is no particular limitation on the compositionratio. Further, the In—Ga—Zn—O-based oxide semiconductor may contain anelement other than In, Ga, and Zn.

Without limitation to the two-layer structure in which the secondcrystalline oxide semiconductor layer is formed over the firstcrystalline oxide semiconductor layer, a stacked structure includingthree or more layers may be formed by repeatedly performing a process ofdeposition and heat treatment for forming a third crystalline oxidesemiconductor layer after the second crystalline oxide semiconductorlayer is formed.

The oxide semiconductor layer 453 including the stack of the oxidesemiconductor layers formed by the above formation method can be used asappropriate for a transistor (e.g., the transistor 162, the transistor410, the transistor 420, the transistor 430, the transistor 441, and thetransistor 442 in Embodiment 1 and Embodiment 2) which can be applied toa semiconductor device disclosed in this specification.

In the transistor 162 of Embodiment 2 in which the stack of the oxidesemiconductor layers of this embodiment is used as the oxidesemiconductor layer 403, an electric field is not applied from onesurface to the other surface of the oxide semiconductor layer andcurrent does not flow in the thickness direction (from one surface tothe other surface; specifically, in the vertical direction in thetransistor 162 illustrated in FIGS. 17A and 17B) of the stack of theoxide semiconductor layers. The transistor has a structure in whichcurrent mainly flows along the interface of the stack of the oxidesemiconductor layers; therefore, even when the transistor is irradiatedwith light or even when a BT stress is applied to the transistor,deterioration of transistor characteristics is suppressed or reduced.

By forming a transistor with the use of a stack of a first crystallineoxide semiconductor layer and a second crystalline oxide semiconductorlayer, like the oxide semiconductor layer 453, the transistor can havestable electric characteristics and high reliability.

This embodiment can be implemented in an appropriate combination withany of the structures described in the other embodiments.

[Embodiment 4]

In this embodiment, an oxide including a crystal with c-axis alignment(also referred to as C-Axis Aligned Crystal (CAAC)), which has atriangular or hexagonal atomic arrangement when seen from the directionof an a-b plane, a surface, or an interface will be described. In thecrystal, metal atoms are arranged in a layered manner, or metal atomsand oxygen atoms are arranged in a layered manner along the c-axis, andthe direction of the a-axis or the b-axis is varied in the a-b plane(the crystal rotates around the c-axis).

In a broad sense, an oxide including CAAC means a non-single crystaloxide including a phase which has a triangular, hexagonal, regulartriangular, or regular hexagonal atomic arrangement when seen from thedirection perpendicular to the a-b plane and in which metal atoms arearranged in a layered manner or metal atoms and oxygen atoms arearranged in a layered manner when seen from the direction perpendicularto the c-axis direction.

The CAAC is not a single crystal, but this does not mean that the CAACis composed of only an amorphous component. Although the CAAC includes acrystallized portion (crystalline portion), a boundary between onecrystalline portion and another crystalline portion is not clear in somecases.

In the case where oxygen is included in the CAAC, nitrogen may besubstituted for part of oxygen included in the CAAC. The c-axes ofindividual crystalline portions included in the CAAC may be aligned inone direction (e.g., a direction perpendicular to a surface of asubstrate over which the CAAC is formed or a surface of the CAAC).Alternatively, the normals of the a-b planes of the individualcrystalline portions included in the CAAC may be aligned in onedirection (e.g., a direction perpendicular to a surface of a substrateover which the CAAC is formed or a surface of the CAAC).

The CAAC becomes a conductor, a semiconductor, or an insulator dependingon its composition or the like. The CAAC transmits or does not transmitvisible light depending on its composition or the like.

As an example of such a CAAC, there is a crystal which is formed into afilm shape and has a triangular or hexagonal atomic arrangement whenobserved from the direction perpendicular to a surface of the film or asurface of a supporting substrate, and in which metal atoms are arrangedin a layered manner or metal atoms and oxygen atoms (or nitrogen atoms)are arranged in a layered manner when a cross section of the film isobserved.

An example of a crystal structure of the CAAC will be described indetail with reference to FIGS. 27A to 27E, FIGS. 28A to 28C, and FIGS.29A to 29C. In FIGS. 27A to 27E, FIGS. 28A to 28C, and FIGS. 29A to 29C,the vertical direction corresponds to the c-axis direction and a planeperpendicular to the c-axis direction corresponds to the a-b plane,unless otherwise specified. When the expressions “an upper half” and “alower half” are simply used, they refer to an upper half above the a-bplane and a lower half below the a-b plane (an upper half and a lowerhalf with respect to the a-b plane). Furthermore, in FIGS. 27A to 27E, Osurrounded by a circle represents tetracoodianate O and O surrounded bya double circle represents tricoodenate O.

FIG. 27A illustrates a structure including one hexacoordinate In atomand six tetracoordinate oxygen (hereinafter referred to astetracoordinate O) atoms proximate to the In atom. Here, a structureincluding one metal atom and oxygen atoms proximate thereto is referredto as a small group. The structure in FIG. 27A is actually an octahedralstructure, but is illustrated as a planar structure for simplicity. Notethat three tetracoordinate O atoms exist in each of an upper half and alower half in FIG. 27A. In the small group illustrated in FIG. 27A,electric charge is 0.

FIG. 27B illustrates a structure including one pentacoordinate Ga atom,three tricoordinate oxygen (hereinafter referred to as tricoordinate O)atoms proximate to the Ga atom, and two tetracoordinate O atomsproximate to the Ga atom. All the tricoordinate O atoms exist on the a-bplane. One tetracoordinate O atom exists in each of an upper half and alower half in FIG. 27B. An In atom can also have the structureillustrated in FIG. 27B because an In atom can have five ligands. In thesmall group illustrated in FIG. 27B, electric charge is 0.

FIG. 27C illustrates a structure including one tetracoordinate Zn atomand four tetracoordinate O atoms proximate to the Zn atom. In FIG. 27C,one tetracoordinate O atom exists in an upper half and threetetracoordinate O atoms exist in a lower half. Alternatively, threetetracoordinate O atoms may exist in the upper half and onetetracoordinate O atom may exist in the lower half in FIG. 27C. In thesmall group illustrated in FIG. 27C, electric charge is 0.

FIG. 27D illustrates a structure including one hexacoordinate Sn atomand six tetracoordinate O atoms proximate to the Sn atom. In FIG. 27D,three tetracoordinate O atoms exist in each of an upper half and a lowerhalf. In the small group illustrated in FIG. 27D, electric charge is +1.

FIG. 27E illustrates a small group including two Zn atoms. In FIG. 27E,one tetracoordinate O atom exists in each of an upper half and a lowerhalf In the small group illustrated in FIG. 27E, electric charge is −1.

Here, a plurality of small groups form a medium group, and a pluralityof medium groups form a large group (also referred to as a unit cell).

Now, a rule of bonding between the small groups will be described. Thethree O atoms in the upper half with respect to the hexacoordinate Inatom in FIG. 27A each have three proximate In atoms in the downwarddirection, and the three O atoms in the lower half each have threeproximate In atoms in the upward direction. The one O atom in the upperhalf with respect to the pentacoordinate Ga atom has one proximate Gaatom in the downward direction, and the one O atom in the lower half hasone proximate Ga atom in the upward direction. The one O atom in theupper half with respect to the tetracoordinate Zn atom has one proximateZn atom in the downward direction, and the three O atoms in the lowerhalf each have three proximate Zn atoms in the upward direction. In thismanner, the number of the tetracoordinate O atoms proximate to and abovethe metal atom is equal to the number of the metal atoms proximate toand below each of the tetracoordinate O atoms. Similarly, the number ofthe tetracoordinate O atoms proximate to and below the metal atom isequal to the number of the metal atoms proximate to and above each ofthe tetracoordinate O atoms. Since O atoms contributing the bindingbetween the small groups are the tetracoordinate O atoms, the sum of thenumber of the metal atoms proximate to and below the O atom and thenumber of the metal atoms proximate to and above the O atom is 4.Accordingly, when the sum of the number of tetracoordinate O atoms abovea metal atom and the number of tetracoordinate O atoms below anothermetal atom is 4, the two kinds of small groups including the metal atomscan be bonded. For example, in the case where the hexacoordinate metal(In or Sn) atom is bonded through three tetracoordinate O atoms in thelower half, it is bonded to the pentacoordinate metal (Ga or In) atom orthe tetracoordinate metal (Zn) atom.

A metal atom having the above coordination number is bonded to anothermetal atom having the above coordination number through atetracoordinate O atom in the c-axis direction. In addition to theabove, a medium group can be formed in a different manner by combining aplurality of small groups so that the total electric charge of thelayered structure is 0.

FIG. 28A illustrates a model of a medium group included in a layeredstructure of an In—Sn—Zn—O-based material. FIG. 28B illustrates a largegroup including three medium groups. Note that FIG. 28C illustrates anatomic arrangement in the case where the layered structure in FIG. 28Bis observed from the c-axis direction.

In FIG. 28A, a tricoordinate O atom is omitted for simplicity, and atetracoordinate O atoms are illustrated by a circle; the number in thecircle shows the number of tetracoordinate O atoms. For example, threetetracoordinate O atoms existing in each of an upper half and a lowerhalf with respect to a Sn atom is denoted by circled 3. Similarly, inFIG. 28A, one tetracoordinate O atom existing in each of an upper halfand a lower half with respect to an In atom is denoted by circled 1.FIG. 28A also illustrates a Zn atom proximate to one tetracoordinate Oatom in a lower half and three tetracoordinate O atoms in an upper half,and a Zn atom proximate to one tetracoordinate O atom in an upper halfand three tetracoordinate O atoms in a lower half.

In the medium group included in the layered structure of theIn—Sn—Zn—O-based material in FIG. 28A, in the order starting from thetop, a Sn atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to an In atom proximate to onetetracoordinate O atom in each of an upper half and a lower half, the Inatom is bonded to a Zn atom proximate to three tetracoordinate O atomsin an upper half, the Zn atom is bonded to an In atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to theZn atom, the In atom is bonded to a small group that includes two Znatoms and is proximate to one tetracoordinate O atom in an upper half,and the small group is bonded to a Sn atom proximate to threetetracoordinate O atoms in each of an upper half and a lower halfthrough one tetracoordinate O atom in a lower half with respect to thesmall group. A plurality of such medium groups are bonded, so that alarge group is formed.

Here, electric charge for one bond of a tricoordinate O atom andelectric charge for one bond of a tetracoordinate O atom can be assumedto be −0.667 and −0.5, respectively. For example, electric charge of a(hexacoordinate or pentacoordinate) In atom, electric charge of a(tetracoordinate) Zn atom, and electric charge of a (pentacoordinate orhexacoordinate) Sn atom are +3, +2, and +4, respectively. Accordingly,electric charge in a small group including a Sn atom is +1. Therefore,electric charge of −1, which cancels +1, is needed to form a layeredstructure including a Sn atom. As a structure having electric charge of−1, the small group including two Zn atoms as illustrated in FIG. 27Ecan be given. For example, with one small group including two Zn atoms,electric charge of one small group including a Sn atom can be cancelled,so that the total electric charge of the layered structure can be 0.

When the large group illustrated in FIG. 28B is repeated, anIn—Sn—Zn—O-based crystal (In₂SnZn₃O₈) can be obtained. Note that alayered structure of the obtained In—Sn—Zn—O-based crystal can beexpressed as a composition formula, In₂SnZn₂O₇(ZnO)_(m) (m is 0 or anatural number).

The above-described rule also applies to the following material: afour-component metal oxide such as an In—Sn—Ga—Zn—O-based material; athree-component metal oxide such as an In—Ga—Zn—O-based material (alsoreferred to as IGZO), an In—Al—Zn—O-based material, a Sn—Ga—Zn—O-basedmaterial, an Al—Ga—Zn—O-based material, a Sn—Al—Zn—O-based material, anIn—Hf—Zn—O-based material, an In—La—Zn—O-based material, anIn—Ce—Zn—O-based material, an In—Pr—Zn—O-based material, anIn—Nd—Zn—O-based material, an In—Sm—Zn—O-based material, anIn—Eu—Zn—O-based material, an In—Gd—Zn—O-based material, anIn—Tb—Zn—O-based material, an In—Dy—Zn—O-based material, anIn—Ho—Zn—O-based material, an In—Er—Zn—O-based material, anIn—Tm—Zn—O-based material, an In—Yb—Zn—O-based material, or anIn—Lu—Zn—O-based material; a two-component metal oxide such as anIn—Zn—O-based material, a Sn—Zn—O-based material, an Al—Zn—O-basedmaterial, a Zn—Mg—O-based material, a Sn—Mg—O-based material, anIn—Mg—O-based material, or an In—Ga—O-based material; and the like.

As an example, FIG. 29A illustrates a model of a medium group includedin a layered structure of an In—Ga—Zn—O-based material.

In the medium group included in the layered structure of theIn—Ga—Zn—O-based material in FIG. 29A, in the order starting from thetop, an In atom proximate to three tetracoordinate O atoms in each of anupper half and a lower half is bonded to a Zn atom proximate to onetetracoordinate O atom in an upper half, the Zn atom is bonded to a Gaatom proximate to one tetracoordinate O atom in each of an upper halfand a lower half through three tetracoordinate O atoms in a lower halfwith respect to the Zn atom, and the Ga atom is bonded to an In atomproximate to three tetracoordinate O atoms in each of an upper half anda lower half through one tetracoordinate O atom in a lower half withrespect to the Ga atom. A plurality of such medium groups are bonded, sothat a large group is formed.

FIG. 29B illustrates a large group including three medium groups. Notethat FIG. 29C illustrates an atomic arrangement in the case where thelayered structure in FIG. 29B is observed from the c-axis direction.

Here, since electric charge of a (hexacoordinate or pentacoordinate) Inatom, electric charge of a (tetracoordinate) Zn atom, and electriccharge of a (pentacoordinate) Ga atom are +3, +2, +3, respectively,electric charge of a small group including any of an In atom, a Zn atom,and a Ga atom is 0. As a result, the total electric charge of a mediumgroup having a combination of such small groups is always 0.

In order to form the layered structure of the In—Ga—Zn—O-based material,a large group can be foamed using not only the medium group illustratedin FIG. 29A but also a medium group in which the arrangement of the Inatom, the Ga atom, and the Zn atom is different from that in FIG. 29A.

[Embodiment 5]

In this embodiment, the field-effect mobility of a transistor will bedescribed.

The actually measured field-effect mobility of an insulated gatetransistor can be lower than its original mobility because of a varietyof reasons; this phenomenon occurs not only in the case of using anoxide semiconductor. One of the reasons that reduce the mobility is adefect inside a semiconductor or a defect at an interface between thesemiconductor and an insulating film. When a Levinson model is used, thefield-effect mobility on the assumption that no defect exists inside thesemiconductor can be calculated theoretically.

Assuming that the original mobility and the measured field-effectmobility of a semiconductor are μ₀ and μ, respectively, and a potentialbarrier (such as a grain boundary) exists in the semiconductor, themeasured field-effect mobility can be expressed as follows.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 2} \rbrack\mspace{585mu}} & \; \\{\mu = {\mu_{0}{\exp( {- \frac{E}{kT}} )}}} & (2)\end{matrix}$

Here, E represents the height of the potential barrier, k represents theBoltzmann constant, and T represents the absolute temperature. When thepotential barrier is assumed to be attributed to a defect, the height ofthe potential barrier can be expressed as follows according to theLevinson model.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 3} \rbrack\mspace{585mu}} & \; \\{E = {\frac{e^{2}N^{2}}{8\; ɛ\; n} = \frac{e^{3}N^{2}t}{8{ɛC}_{ox}V_{g}}}} & (3)\end{matrix}$

Here, e represents the elementary charge, N represents the averagedefect density per unit area in a channel, ∈ represents the permittivityof the semiconductor, n represents the number of carriers per unit areain the channel, C_(ox) represents the capacitance per unit area, V_(g)represents the gate voltage, and t represents the thickness of thechannel. In the case where the thickness of the semiconductor layer isless than or equal to 30 nm, the thickness of the channel may beregarded as being the same as the thickness of the semiconductor layer.The drain current I_(d) in a linear region can be expressed as follows.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 4} \rbrack\mspace{585mu}} & \; \\{I_{d} = {\frac{W_{\mu}V_{g}V_{d}C_{ox}}{L}{\exp( {- \frac{E}{kT}} )}}} & (4)\end{matrix}$

Here, L represents the channel length and W represents the channelwidth, and L and W are each 10 μm. In addition, V_(d) represents thedrain voltage. When dividing both sides of the above equation by V_(g)and then taking logarithms of both sides, the following formula can beobtained.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 5} \rbrack\mspace{585mu}} & \; \\{{\ln( \frac{I_{d}}{V_{g}} )} = {{{\ln( \frac{W_{\mu}V_{d}C_{ox}}{L} )} - \frac{E}{kT}} = {{\ln( \frac{W_{\mu}V_{d}C_{ox}}{L} )} - \frac{e^{3}N^{2}t}{8{kT}\;{ɛC}_{ox}V_{g}}}}} & (5)\end{matrix}$

The right side of Formula 5 is a function of V_(g). From the formula, itis found that the defect density N can be obtained from the slope of aline in a graph which is obtained by plotting actual measured valueswith In(I_(d)/V_(g)) as the ordinate and 1/V_(g) as the abscissa. Thatis, the defect density can be evaluated from the I_(d)-V_(g)characteristics of the transistor. The defect density N of an oxidesemiconductor in which the ratio of indium (In), tin (Sn), and zinc (Zn)is 1:1:1 is approximately 1×10¹²/cm².

On the basis of the defect density obtained in this manner, or the like,μ₀ can be calculated to be 120 cm²/Vs from Formula (2) and Formula (3).The measured mobility of an In—Sn—Zn oxide including a defect isapproximately 40 cm²/Vs. However, assuming that no defect exists insidethe semiconductor and at the interface between the semiconductor and aninsulating film, the mobility μ₀ of the oxide semiconductor is expectedto be 120 cm²/Vs.

Note that even when no defect exists inside a semiconductor, scatteringat an interface between a channel and a gate insulating layer affectsthe transport property of the transistor. In other words, the mobilityμ₁ at a position that is distance x away from the interface between thechannel and the gate insulating layer can be expressed as follows.

$\begin{matrix}{\lbrack {{FORMULA}\mspace{14mu} 6} \rbrack\mspace{585mu}} & \; \\{\frac{1}{\mu_{1}} = {\frac{1}{\mu_{0}} + {\frac{D}{B}{\exp( {- \frac{x}{l}} )}}}} & (6)\end{matrix}$

Here, D represents the electric field in the gate direction, and B and lare constants. B and l can be obtained from actual measurement results;according to the above measurement results, B is 4.75×10⁷ cm/s and l is10 nm (the depth to which the influence of interface scatteringreaches). When D is increased (i.e., when the gate voltage isincreased), the second term of Formula (6) is increased and accordinglythe mobility μ₁ is decreased.

Calculation results of the mobility μ₂ of a transistor whose channelincludes an ideal oxide semiconductor without a defect inside thesemiconductor are shown in FIG. 30. For the calculation, devicesimulation software Sentaurus Device manufactured by Synopsys, Inc. wasused, and the bandgap, the electron affinity, the relative permittivity,and the thickness of the oxide semiconductor were assumed to be 2.8 eV,4.7 eV, 15, and 15 nm, respectively. These values were obtained bymeasurement of a thin film that was formed by a sputtering method.

Further, the work functions of a gate, a source, and a drain wereassumed to be 5.5 eV, 4.6 eV, and 4.6 eV, respectively. The thickness ofa gate insulating layer was assumed to be 100 nm, and the relativepermittivity thereof was assumed to be 4.1. The channel length and thechannel width were each assumed to be 10 μm, and the drain voltage V_(d)was assumed to be 0.1 V.

As shown in FIG. 30, the mobility has a peak of more than 100 cm²/Vs ata gate voltage that is a little over 1 V and is decreased as the gatevoltage becomes higher because the influence of interface scattering isincreased. Note that in order to reduce interface scattering, it isdesirable that a surface of the semiconductor layer be flat at theatomic level (atomic layer flatness).

Calculation results of characteristics of minute transistors which aremanufactured using an oxide semiconductor having such a mobility areshown in FIGS. 31A to 31C, FIGS. 32A to 32C, and FIGS. 33A to 33C. FIGS.34A and 34B illustrate cross-sectional structures of the transistorsused for the calculation. The transistors illustrated in FIGS. 34A and34B each include a semiconductor region 1103 a and a semiconductorregion 1103 c which have n⁺-type conductivity in an oxide semiconductorlayer. The resistivities of the semiconductor region 1103 a and thesemiconductor region 1103 c are 2×10⁻³ Ωcm.

The transistor illustrated in FIG. 34A is formed over a base insulatinglayer 1101 and an embedded insulator 1102 which is embedded in the baseinsulating layer 1101 and formed of aluminum oxide. The transistorincludes the semiconductor region 1103 a, the semiconductor region 1103c, an intrinsic semiconductor region 1103 b serving as a channelformation region therebetween, and a gate electrode 1105. The width ofthe gate electrode 1105 is 33 nm.

A gate insulating layer 1104 is formed between the gate electrode 1105and the semiconductor region 1103 b. In addition, a sidewall insulatinglayer 1106 a and a sidewall insulating layer 1106 b are formed on bothside surfaces of the gate electrode 1105, and an insulating layer 1107is formed over the gate electrode 1105 so as to prevent a short circuitbetween the gate electrode 1105 and another wiring. The sidewallinsulating layer has a width of 5 nm. A source electrode 1108 a and adrain electrode 1108 b are provided in contact with the semiconductorregion 1103 a and the semiconductor region 1103 c, respectively. Notethat the channel width of this transistor is 40 nm.

The transistor of FIG. 34B is the same as the transistor of FIG. 34A inthat it is formed over the base insulating layer 1101 and the embeddedinsulator 1102 formed of aluminum oxide and that it includes thesemiconductor region 1103 a, the semiconductor region 1103 c, theintrinsic semiconductor region 1103 b provided therebetween, the gateelectrode 1105 having a width of 33 nm, the gate insulating layer 1104,the sidewall insulating layer 1106 a, the sidewall insulating layer 1106b, the insulating layer 1107, the source electrode 1108 a, and the drainelectrode 1108 b.

The transistor illustrated in FIG. 34A is different from the transistorillustrated in FIG. 34B in the conductivity type of semiconductorregions under the sidewall insulating layer 1106 a and the sidewallinsulating layer 1106 b. In the transistor illustrated in FIG. 34A, thesemiconductor regions under the sidewall insulating layer 1106 a and thesidewall insulating layer 1106 b are part of the semiconductor region1103 a having n⁺-type conductivity and part of the semiconductor region1103 c having n⁺-type conductivity, whereas in the transistorillustrated in FIG. 34B, the semiconductor regions under the sidewallinsulating layer 1106 a and the sidewall insulating layer 1106 b arepart of the intrinsic semiconductor region 1103 b. In other words, inthe semiconductor layer of FIG. 34B, a region having a width of Loffwhich overlaps with neither the semiconductor region 1103 a (thesemiconductor region 1103 c) nor the gate electrode 1105 is provided.This region is called an offset region, and the width Loff is called anoffset length. As is seen from the drawing, the offset length is equalto the width of the sidewall insulating layer 1106 a (the sidewallinsulating layer 1106 b).

The other parameters used in calculation are as described above. For thecalculation, device simulation software Sentaurus Device manufactured bySynopsys, Inc. was used. FIGS. 31A to 31C show the gate voltage (V_(g):a potential difference between the gate and the source) dependence ofthe drain current (I_(d), a solid line) and the mobility (μ, a dottedline) of the transistor having the structure illustrated in FIG. 34A.The drain current I_(d) is obtained by calculation under the assumptionthat the drain voltage (a potential difference between the drain and thesource) is +1 V and the mobility μ is obtained by calculation under theassumption that the drain voltage is +0.1 V.

FIG. 31A shows the gate voltage dependence of the transistor in the casewhere the thickness of the gate insulating film is 15 nm, FIG. 31B showsthat of the transistor in the case where the thickness of the gateinsulating layer is 10 nm, and FIG. 31C shows that of the transistor inthe case where the thickness of the gate insulating layer is 5 nm. Asthe gate insulating film is thinner, the drain current I_(d) (off-statecurrent) particularly in an off state is significantly decreased. Incontrast, there is no noticeable change in the peak value of themobility μ and the drain current I_(d) in an on state (on-statecurrent). The graphs show that the drain current exceeds 10 μA, which isrequired in a memory element and the like, at a gate voltage of around 1V.

FIGS. 32A to 32C show the gate voltage V_(g) dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 34B where the offsetlength Loff is 5 nm. The drain current I_(d) is obtained by calculationunder the assumption that the drain voltage is +1 V and the mobility μis obtained by calculation under the assumption that the drain voltageis +0.1 V. FIG. 32A shows the gate voltage dependence of the transistorin the case where the thickness of the gate insulating film is 15 nm,FIG. 32B shows that of the transistor in the case where the thickness ofthe gate insulating layer is 10 nm, and FIG. 32C shows that of thetransistor in the case where the thickness of the gate insulating layeris 5 nm.

Further, FIGS. 33A to 33C show the gate voltage dependence of the draincurrent I_(d) (a solid line) and the mobility μ (a dotted line) of thetransistor having the structure illustrated in FIG. 34B where the offsetlength Loff is 15 nm. The drain current I_(d) is obtained by calculationunder the assumption that the drain voltage is +1 V and the mobility μis obtained by calculation under the assumption that the drain voltageis +0.1 V. FIG. 33A shows the gate voltage dependence of the transistorin the case where the thickness of the gate insulating film is 15 nm,FIG. 33B shows that of the transistor in the case where the thickness ofthe gate insulating layer is 10 nm, and FIG. 33C shows that of thetransistor in the case where the thickness of the gate insulating layeris 5 nm.

In either of the structures, as the gate insulating layer is thinner,the off-state current is significantly decreased, whereas no noticeablechange arises in the peak value of the mobility μ and the on-statecurrent.

Note that the peak of the mobility μ is approximately 80 cm²/Vs in FIGS.31A to 31C, approximately 60 cm²/Vs in FIGS. 32A to 32C, andapproximately 40 cm²/Vs in FIGS. 33A to 33C; thus, the peak of themobility μ is decreased as the offset length Loff is increased. Further,the same applies to the off-state current. The on-state current is alsodecreased as the offset length Loff is increased; however, the decreasein the on-state current is much more gradual than the decrease in theoff-state current. Further, the graphs show that in either of thestructures, the drain current exceeds 10 μA, which is required in amemory element and the like, at a gate voltage of around 1 V.

[Embodiment 6]

In this embodiment, a transistor in which an oxide semiconductorincluding In, Sn, and Zn as main components is used as an oxidesemiconductor will be described.

A transistor in which an oxide semiconductor including In, Sn, and Zn asmain components is used as a channel formation region can have favorablecharacteristics by depositing the oxide semiconductor while heating asubstrate or by performing heat treatment after an oxide semiconductorfilm is formed. Note that a main component refers to an element includedin a composition at 5 atomic % or more.

By intentionally heating the substrate after deposition of the oxidesemiconductor film including In, Sn, and Zn as main components, thefield-effect mobility of the transistor can be improved. Further, thethreshold voltage of the transistor can be positively shifted to makethe transistor normally off.

As an example, FIGS. 35A to 35C each show characteristics of atransistor in which an oxide semiconductor film including In, Sn, and Znas main components and having a channel length L of 3 μm and a channelwidth W of 10 μm, and a gate insulating film with a thickness of 100 nmare used. Note that V_(d) was set to 10 V.

FIG. 35A shows characteristics of a transistor whose oxide semiconductorfilm including In, Sn, and Zn as main components was formed by asputtering method without heating a substrate intentionally. A peak ofthe field-effect mobility of the transistor is 18.8 cm²/Vsec. On theother hand, when the oxide semiconductor film including In, Sn, and Znas main components is formed while heating the substrate intentionally,the field-effect mobility can be improved. FIG. 35B showscharacteristics of a transistor whose oxide semiconductor film includingIn, Sn, and Zn as main components was formed while heating a substrateat 200° C. A peak of the field-effect mobility of the transistor is 32.2cm²/Vsec.

The field-effect mobility can be further improved by performing heattreatment after formation of the oxide semiconductor film including In,Sn, and Zn as main components. FIG. 35C shows characteristics of atransistor whose oxide semiconductor film including In, Sn, and Zn asmain components was deposited by sputtering at 200° C. and thensubjected to heat treatment at 650° C. A peak of the field-effectmobility of the transistor is 34.5 cm²/Vsec.

The intentional heating of the substrate is expected to have an effectof reducing moisture taken into the oxide semiconductor film during thedeposition by sputtering. Further, the heat treatment after depositionenables hydrogen, a hydroxyl group, or moisture to be released andremoved from the oxide semiconductor film. In this manner, thefield-effect mobility can be improved. Such an improvement infield-effect mobility is presumed to be achieved not only by removal ofimpurities by dehydration or dehydrogenation but also by a reduction ininteratomic distance due to an increase in density. The oxidesemiconductor can be crystallized by being highly purified by removal ofimpurities from the oxide semiconductor. In the case of using such ahighly purified non-single-crystal oxide semiconductor, ideally, a peakof a field-effect mobility exceeding 100 cm²/Vsec is expected to berealized.

The oxide semiconductor including In, Sn, and Zn as main components maybe crystallized in the following manner: oxygen ions are implanted intothe oxide semiconductor, hydrogen, a hydroxyl group, and/or moistureincluded in the oxide semiconductor is released by heat treatment, andthe oxide semiconductor is crystallized through the heat treatment or byanother heat treatment performed later. By such crystallizationtreatment or recrystallization treatment, a non-single-crystal oxidesemiconductor having favorable crystallinity can be obtained.

The intentional heating of the substrate during deposition and/or theheat treatment after the deposition contributes not only to improvingfield-effect mobility but also to making the transistor normally off. Ina transistor in which an oxide semiconductor film that includes In, Sn,and Zn as main components and is formed without heating a substrateintentionally is used as a channel formation region, the thresholdvoltage tends to be shifted negatively. However, when the oxidesemiconductor film formed while heating the substrate intentionally isused, the problem of the negative shift of the threshold voltage can besolved. That is, the threshold voltage is shifted so that the transistorbecomes normally off; this tendency can be confirmed by comparisonbetween FIGS. 35A and 35B.

Note that the threshold voltage can also be controlled by changing theratio of In, Sn, and Zn; when the composition ratio of In, Sn, and Zn is2:1:3, a normally-off transistor is expected to be formed. In addition,an oxide semiconductor film having high crystallinity can be obtained bysetting the composition ratio of a target as follows: In:Sn:Zn=2:1:3.

The temperature of the intentional heating of the substrate or thetemperature of the heat treatment is 150° C. or higher, preferably 200°C. or higher, further preferably 400° C. or higher. When deposition orheat treatment is performed at a high temperature, the transistor can benormally off.

By intentionally heating the substrate during deposition and/or byperforming heat treatment after the deposition, the stability against agate-bias stress can be increased. For example, when a gate bias isapplied with an intensity of 2 MV/cm at 150° C. for one hour, drift ofthe threshold voltage can be less than ±1.5 V, preferably less than ±1.0V.

A BT test was performed on the following two transistors: Sample 1 onwhich heat treatment was not performed after formation of an oxidesemiconductor film, and Sample 2 on which heat treatment at 650° C. wasperformed after deposition of an oxide semiconductor film.

First, V_(g)-I_(d) characteristics of the transistors were measured at asubstrate temperature of 25° C. and V_(ds) of 10 V. Note that V_(ds)refers to a drain voltage (a potential difference between a drain and asource). Then, the substrate temperature was set to 150° C. and V_(ds)was set to 0.1 V. After that, 20 V of V_(g) was applied so that theintensity of an electric field applied to gate insulating films was 2MV/cm, and the condition was kept for one hour. Next, V_(g) was set to 0V. Then, V_(g)-I_(d) characteristics of the transistors were measured ata substrate temperature of 25° C. and V_(ds) of 10 V. This process iscalled a positive BT test.

In a similar manner, first, V_(g)-I_(d) characteristics of thetransistors were measured at a substrate temperature of 25° C. andV_(ds) of 10 V. Then, the substrate temperature was set at 150° C. andV_(ds) was set to 0.1 V. After that, −20 V of V_(g) was applied so thatthe intensity of an electric field applied to the gate insulating filmswas −2 MV/cm, and the condition was kept for one hour. Next, V_(g) wasset to 0 V. Then, V_(g)-I_(d) characteristics of the transistors weremeasured at a substrate temperature of 25° C. and V_(ds) of 10 V. Thisprocess is called a negative BT test.

FIGS. 36A and 36B show a result of the positive BT test of Sample 1 anda result of the negative BT test of Sample 1, respectively. FIGS. 37Aand 37B show a result of the positive BT test of Sample 2 and a resultof the negative BT test of Sample 2, respectively.

The amount of shift in the threshold voltage of Sample 1 due to thepositive BT test and that due to the negative BT test were 1.80 V and−0.42 V, respectively. The amount of shift in the threshold voltage ofSample 2 due to the positive BT test and that due to the negative BTtest were 0.79 V and 0.76 V, respectively. It is found that, in each ofSample 1 and Sample 2, the amount of shift in the threshold voltagebetween before and after the BT tests is small and the reliabilitythereof is high.

The heat treatment can be performed in an oxygen atmosphere;alternatively, the heat treatment may be performed first in anatmosphere of nitrogen or an inert gas or under reduced pressure, andthen in an atmosphere including oxygen. By performing the heat treatmentunder the condition, oxygen can be excessively supplied to the oxidesemiconductor film. Oxygen is supplied to the oxide semiconductor filmafter dehydration or dehydrogenation, whereby an effect of the heattreatment can be further increased. As a method for supplying oxygenafter dehydration or dehydrogenation, a method in which oxygen ions areaccelerated by an electric field and implanted into the oxidesemiconductor film may be employed. Thus, oxygen can be also excessivelysupplied to the oxide semiconductor film.

A defect due to oxygen deficiency is easily caused in the oxidesemiconductor or at an interface between the oxide semiconductor and afilm in contact with the oxide semiconductor; however, when excessoxygen is included in the oxide semiconductor by the heat treatment,oxygen deficiency caused later can be compensated for with excessoxygen. The excess oxygen is oxygen existing mainly between lattices.When the concentration of excess oxygen is set to higher than or equalto 1×10¹⁶/cm³ and lower than or equal to 2×10²⁰/cm³, excess oxygen canbe included in the oxide semiconductor without causing crystaldistortion or the like.

When heat treatment is performed so that at least part of the oxidesemiconductor includes crystal, a more stable oxide semiconductor filmcan be obtained. For example, when an oxide semiconductor film which isformed by sputtering using a target having a composition ratio ofIn:Sn:Zn=1:1:1 without heating a substrate intentionally is analyzed byX-ray diffraction (XRD), a halo pattern is observed. The formed oxidesemiconductor film can be crystallized by being subjected to heattreatment. The temperature of the heat treatment can be set asappropriate; when the heat treatment is performed at 650° C., forexample, a clear diffraction peak can be observed in an X-raydiffraction analysis.

An XRD analysis of an In—Sn—Zn—O film was conducted. The XRD analysiswas conducted using an X-ray diffractometer D8 ADVANCE manufactured byBruker AXS, and measurement was performed by an out-of-plane method.

Sample A and Sample B were prepared and the XRD analysis was performedthereon. A method for manufacturing Sample A and Sample B will bedescribed below.

An In—Sn—Zn—O film with a thickness of 100 nm was formed over a quartzsubstrate that had been subjected to dehydrogenation treatment.

The In—Sn—Zn—O film was formed with a sputtering apparatus with a powerof 100 W (DC) in an oxygen atmosphere. An In—Sn—Zn—O target having anatomic ratio of In:Sn:Zn=1:1:1 was used as a target. Note that thesubstrate heating temperature in deposition was set at 200° C. A samplemanufactured in this manner was used as Sample A.

Next, a sample manufactured by a method similar to that of Sample A wassubjected to heat treatment at 650° C. As the heat treatment, heattreatment in a nitrogen atmosphere was first performed for one hour andheat treatment in an oxygen atmosphere was further performed for onehour without lowering the temperature. A sample manufactured in thismanner was used as Sample B.

FIG. 40 shows XRD spectra of Sample A and Sample B. No peak derived fromcrystal was observed in Sample A, whereas peaks derived from crystalwere observed when 2θwas around 35 deg. and at 37 deg. to 38 deg. inSample B.

As described above, by intentionally heating a substrate duringdeposition of an oxide semiconductor including In, Sn, and Zn as maincomponents and/or by performing heat treatment after the deposition,characteristics of a transistor can be improved.

These substrate heating and heat treatment have an effect of preventinghydrogen and a hydroxyl group, which are unfavorable impurities for anoxide semiconductor, from being included in the film or an effect ofremoving hydrogen and a hydroxyl group from the film. That is, an oxidesemiconductor can be highly purified by removing hydrogen serving as adonor impurity from the oxide semiconductor, whereby a normally-offtransistor can be obtained. The high purification of an oxidesemiconductor enables the off-state current of the transistor to be 1aA/μm or lower. Here, the unit of the off-state current is used toindicate current per micrometer of a channel width.

FIG. 41 shows a relation between the off-state current of a transistorand the inverse of substrate temperature (absolute temperature) atmeasurement. Here, for simplicity, the horizontal axis represents avalue (1000/T) obtained by multiplying an inverse of substratetemperature at measurement by 1000.

Specifically, as shown in FIG. 41, the off-state current can be 1 aA/μm(1×10⁻¹⁸ A/μm) or lower, 100 zA/μm (1×10⁻¹⁹ A/μm) or lower, and 1 zA/μm(1×10⁻²¹ A/μm) or lower when the substrate temperature is 125° C., 85°C., and room temperature (27° C.), respectively. Preferably, theoff-state current can be 0.1 aA/μm (1×10⁻¹⁹ A/μm) or lower, 10 zA/μm(1×10⁻²⁰ A/μm) or lower, and 0.1 zA/μm (1×10⁻²² A/μm) or lower at 125°C., 85° C., and room temperature, respectively.

Note that in order to prevent hydrogen and moisture from being includedin the oxide semiconductor film during deposition thereof, it ispreferable to increase the purity of a sputtering gas by sufficientlysuppressing leakage from the outside of a deposition chamber anddegasification through an inner wall of the deposition chamber. Forexample, a gas with a dew point of −70° C. or lower is preferably usedas the sputtering gas in order to prevent moisture from being includedin the film. In addition, it is preferable to use a target which ishighly purified so as not to include impurities such as hydrogen andmoisture. Although it is possible to remove moisture from a film of anoxide semiconductor including In, Sn, and Zn as main components by heattreatment, a film which does not include moisture originally ispreferably formed because moisture is released from the oxidesemiconductor including In, Sn, and Zn as main components at a highertemperature than from an oxide semiconductor including In, Ga, and Zn asmain components.

The relation between the substrate temperature and electriccharacteristics of a transistor formed using Sample B, on which heattreatment at 650° C. was performed after deposition of the oxidesemiconductor film, was evaluated.

The transistor used for the measurement has a channel length L of 3 μm,a channel width W of 10 μm, Lov of 0 μm, and dW of 0 μm. Note thatV_(ds) was set to 10 V. Note that the substrate temperature was −40° C.,−25° C., 25° C., 75° C., 125° C., and 150° C. Here, in a transistor, thewidth of a portion where a gate electrode overlaps with one of a pair ofelectrodes is referred to as Lov, and the width of a portion of the pairof electrodes, which does not overlap with an oxide semiconductor film,is referred to as dW.

FIG. 38 shows the V_(g) dependence of I_(d) (a solid line) andfield-effect mobility (a dotted line). FIG. 39A shows a relation betweenthe substrate temperature and the threshold voltage, and FIG. 39B showsa relation between the substrate temperature and the field-effectmobility.

From FIG. 39A, it is found that the threshold voltage gets lower as thesubstrate temperature increases. Note that the threshold voltage isdecreased from 1.09 V to −0.23 V in the range from −40° C. to 150° C.

From FIG. 39B, it is found that the field-effect mobility gets lower asthe substrate temperature increases. Note that the field-effect mobilityis decreased from 36 cm²/Vs to 32 cm²/Vs in the range from −40° C. to150° C. Thus, it is found that variation in electric characteristics issmall in the above temperature range.

In a transistor in which such an oxide semiconductor including In, Sn,and Zn as main components is used as a channel formation region, afield-effect mobility of 30 cm²/Vsec or higher, preferably 40 cm²/Vsecor higher, further preferably 60 cm²/Vsec or higher can be obtained withthe off-state current maintained at 1 aA/μm or lower, which can achieveon-state current needed for an LSI. For example, in an FET where L/W is33 nm/40 nm, an on-state current of 12 μA or higher can flow when thegate voltage is 2.7 V and the drain voltage is 1.0 V. In addition,sufficient electric characteristics can be ensured in a temperaturerange needed for operation of a transistor. With such characteristics,an integrated circuit having a novel function can be realized withoutdecreasing the operation speed even when a transistor including an oxidesemiconductor is also provided in an integrated circuit formed using aSi semiconductor.

Example 1 of Transistor

An example of a transistor in which an In—Sn—Zn—O film is used as anoxide semiconductor film will be described below with reference to FIGS.42A and 42B and the like.

FIGS. 42A and 42B are a top view and a cross-sectional view of acoplanar transistor having a top-gate top-contact structure. FIG. 42A isthe top view of the transistor. FIG. 42B illustrates cross section A-Balong dashed-dotted line A-B in FIG. 42A.

The transistor illustrated in FIG. 42B includes a substrate 1200; a baseinsulating film 1202 provided over the substrate 1200; a protectiveinsulating film 1204 provided in the periphery of the base insulatingfilm 1202; an oxide semiconductor film 1206 provided over the baseinsulating film 1202 and the protective insulating film 1204 andincluding a high-resistance region 1206 a and low-resistance regions1206 b; a gate insulating film 1208 provided over the oxidesemiconductor film 1206; a gate electrode 1210 provided to overlap withthe oxide semiconductor film 1206 with the gate insulating film 1208positioned therebetween; a sidewall insulating film 1212 provided incontact with a side surface of the gate electrode 1210; a pair ofelectrodes 1214 provided in contact with at least the low-resistanceregions 1206 b; an interlayer insulating film 1216 provided to cover atleast the oxide semiconductor film 1206, the gate electrode 1210, andthe pair of electrodes 1214; and a wiring 1218 provided to be connectedto at least one of the pair of electrodes 1214 through an opening formedin the interlayer insulating film 1216.

Although not illustrated, a protective film may be provided to cover theinterlayer insulating film 1216 and the wiring 1218. With the protectivefilm, a minute amount of leakage current generated by surface conductionof the interlayer insulating film 1216 can be reduced and thus theoff-state current of the transistor can be reduced.

Example 2 of Transistor

Another example of a transistor in which an In—Sn—Zn—O film is used asan oxide semiconductor film will be described below.

FIGS. 43A and 43B are a top view and a cross-sectional view whichillustrate a structure of a transistor. FIG. 43A is the top view of thetransistor. FIG. 43B is a cross-sectional view along dashed-dotted lineA-B in FIG. 43A.

The transistor illustrated in FIG. 43B includes a substrate 1600; a baseinsulating film 1602 provided over the substrate 1600; an oxidesemiconductor film 1606 provided over the base insulating film 1602; apair of electrodes 1614 in contact with the oxide semiconductor film1606; a gate insulating film 1608 provided over the oxide semiconductorfilm 1606 and the pair of electrodes 1614; a gate electrode 1610provided to overlap with the oxide semiconductor film 1606 with the gateinsulating film 1608 positioned therebetween; an interlayer insulatingfilm 1616 provided to cover the gate insulating film 1608 and the gateelectrode 1610; wirings 1618 connected to the pair of electrodes 1614through openings formed in the interlayer insulating film 1616; and aprotective film 1620 provided to cover the interlayer insulating film1616 and the wirings 1618.

As the substrate 1600, a glass substrate can be used. As the baseinsulating film 1602, a silicon oxide film can be used. As the oxidesemiconductor film 1606, an In—Sn—Zn—O film can be used. As the pair ofelectrodes 1614, a tungsten film can be used. As the gate insulatingfilm 1608, a silicon oxide film can be used. The gate electrode 1610 canhave a layered structure of a tantalum nitride film and a tungsten film.The interlayer insulating film 1616 can have a layered structure of asilicon oxynitride film and a polyimide film. The wirings 1618 can eachhave a layered structure in which a titanium film, an aluminum film, anda titanium film are formed in this order. As the protective film 1620, apolyimide film can be used.

Note that in the transistor having the structure illustrated in FIG.43A, the width of a portion where the gate electrode 1610 overlaps withone of the pair of electrodes 1614 is referred to as Lov. Similarly, thewidth of a portion of the pair of electrodes 1614, which does notoverlap with the oxide semiconductor film 1606, is referred to as dW.

[Embodiment 7]

In this embodiment, the cases where the semiconductor device describedin any of the above embodiments is applied to an electronic appliancewill be described with reference to FIGS. 23A to 23F. In thisembodiment, applications of the semiconductor device to electronicappliances such as a computer, a cellular phone handset (also referredto as a cellular phone or a cellular phone device), a personal digitalassistant (including a portable game machine, an audio reproducingdevice, and the like), a digital camera, a digital video camera,electronic paper, and a television set (also referred to as a televisionor a television receiver) are described.

FIG. 23A is a laptop personal computer including a housing 701, ahousing 702, a display portion 703, a keyboard 704, and the like. Thesemiconductor device described in any of the above embodiments isprovided in at least one of the housing 701 and the housing 702.Therefore, a laptop personal computer in which writing and reading ofdata are performed at high speed, data is stored for a long time, andpower consumption is sufficiently reduced can be realized.

FIG. 23B is a personal digital assistant (PDA). A main body 711 isprovided with a display portion 713, an external interface 715,operation buttons 714, and the like. Further, a stylus 712 and the likefor operation of the personal digital assistant are provided. In themain body 711, the semiconductor device described in any of the aboveembodiments is provided. Therefore, a personal digital assistant inwhich writing and reading of data are performed at high speed, data isstored for a long time, and power consumption is sufficiently reducedcan be realized.

FIG. 23C is an e-book reader 720 mounted with electronic paper, whichincludes two housings, a housing 721 and a housing 723. The housing 721and the housing 723 are provided with a display portion 725 and adisplay portion 727, respectively. The housings 721 and 723 areconnected by a hinge portion 737 and can be opened or closed with thehinge portion 737. The housing 721 is provided with a power supply 731,an operation key 733, a speaker 735, and the like. At least one of thehousings 721 and 723 is provided with the semiconductor device describedin any of the above embodiments. Therefore, an e-book reader in whichwriting and reading of data are performed at high speed, data is storedfor a long time, and power consumption is sufficiently reduced can berealized.

FIG. 23D is a cellular phone including two housings, a housing 740 and ahousing 741. Moreover, the housings 740 and 741 which are shown unfoldedin FIG. 23D can overlap with each other by sliding; thus, the size ofthe cellular phone can be reduced, which makes the cellular phonesuitable for being carried. The housing 741 includes a display panel742, a speaker 743, a microphone 744, an operation key 745, a pointingdevice 746, a camera lens 747, an external connection terminal 748, andthe like. The housing 740 includes a solar cell 749 for charging thecellular phone, an external memory slot 750, and the like. In addition,an antenna is incorporated in the housing 741. At least one of thehousings 740 and 741 is provided with the semiconductor device describedin any of the above embodiments. Therefore, a cellular phone in whichwriting and reading of data are performed at high speed, data is storedfor a long time, and power consumption is sufficiently reduced can berealized.

FIG. 23E is a digital camera including a main body 761, a displayportion 767, an eyepiece 763, an operation switch 764, a display portion765, a battery 766, and the like. In the main body 761, thesemiconductor device described in any of the above embodiments isprovided. Therefore, a digital camera in which writing and reading ofdata are performed at high speed, data is stored for a long time, andpower consumption is sufficiently reduced can be realized.

FIG. 23F is a television device 770 including a housing 771, a displayportion 773, a stand 775, and the like. The television device 770 can beoperated with an operation switch of the housing 771 or a remotecontroller 780. The semiconductor device described in any of the aboveembodiments is mounted on the housing 771 and the remote controller 780.Therefore, a television set in which writing and reading of data areperformed at high speed, data is stored for a long time, and powerconsumption is sufficiently reduced can be realized.

As described above, the electronic appliances described in thisembodiment each include the semiconductor device described in any of theabove embodiments; thus, electronic appliances with low powerconsumption can be realized.

This application is based on Japanese Patent Application serial no.2010-178169 filed with Japan Patent Office on Aug. 6, 2010 and JapanesePatent Application serial no. 2011-107864 filed with Japan Patent Officeon May 13, 2011, the entire contents of which are hereby incorporated byreference.

What is claimed is:
 1. A semiconductor device comprising: a wiring; amemory cell comprising: a first transistor including a first gateelectrode, a first source electrode, a first drain electrode, and afirst channel formation region; and a second transistor including asecond gate electrode, a second source electrode, a second drainelectrode, and a second channel formation region; and a reading circuitcomprising a load, a clocked inverter, and a third transistor, wherein amaterial of the first channel formation region is different from that ofthe second channel formation region, wherein the third transistorcomprises a third gate electrode, a third source electrode, a thirddrain electrode, and a third channel formation region, wherein an inputterminal of the clocked inverter is electrically connected to the loadand one of the first source electrode and the first drain electrodethrough the wiring, wherein an output terminal of the clocked inverteris electrically connected to one of the third source electrode and thethird drain electrode, and wherein the other of the third sourceelectrode and the third drain electrode is electrically connected to apower supply potential.
 2. The semiconductor device according to claim1, wherein the first transistor is a p-channel transistor, and whereinthe second transistor is an n-channel transistor.
 3. The semiconductordevice according to claim 1, wherein the second channel formation regionincludes an oxide semiconductor.
 4. The semiconductor device accordingto claim 1, further comprising a latch circuit electrically connected tothe output terminal of the clocked inverter.
 5. The semiconductor deviceaccording to claim 1, wherein the wiring is electrically connected toone of the second source electrode and the second drain electrode.
 6. Asemiconductor device comprising: a first wiring; a second wiring; amemory cell comprising: a first transistor including a first gateelectrode, a first source electrode, a first drain electrode, and afirst channel formation region; a second transistor including a secondgate electrode, a second source electrode, a second drain electrode, anda second channel formation region; and a capacitor, wherein a firstelectrode of the capacitor is electrically connected to the first gateelectrode and one of the second source electrode and the second drainelectrode and wherein a second electrode of the capacitor iselectrically connected to the second wiring; and a reading circuitcomprising a load, a clocked inverter, and a third transistor, wherein amaterial of the first channel formation region is different from that ofthe second channel formation region, wherein the third transistorcomprises a third gate electrode, a third source electrode, a thirddrain electrode, and a third channel formation region, wherein an inputterminal of the clocked inverter is electrically connected to the loadand one of the first source electrode and the first drain electrodethrough the first wiring, wherein an output terminal of the clockedinverter is connected to one of the third source electrode and the thirddrain electrode, wherein the other of the third source electrode and thethird drain electrode is electrically connected to a power supplypotential, wherein the third transistor is configured to be turned OFFwhenever a potential of the second wiring is changed, and wherein aclock signal having a high-level potential is input to a clock signalline in the clocked inverter whenever the potential of the second wiringis changed.
 7. The semiconductor device according to claim 6, whereinthe first transistor is a p-channel transistor, and wherein the secondtransistor is an n-channel transistor.
 8. The semiconductor deviceaccording to claim 6, wherein the second channel formation regionincludes an oxide semiconductor.
 9. The semiconductor device accordingto claim 6, further comprising a latch circuit electrically connected tothe output terminal of the clocked inverter.
 10. The semiconductordevice according to claim 6, wherein the first wiring is electricallyconnected to one of the second source electrode and the second drainelectrode.
 11. A driving method of a semiconductor device comprising: afirst wiring; a second wiring; a memory cell comprising: a firsttransistor including a first gate electrode, a first source electrode, afirst drain electrode, and a first channel formation region; a secondtransistor including a second gate electrode, a second source electrode,a second drain electrode, and a second channel formation region; and acapacitor, wherein a first electrode of the capacitor is electricallyconnected to the first gate electrode and one of the second sourceelectrode and the second drain electrode and wherein a second electrodeof the capacitor is electrically connected to the second wiring; and areading circuit comprising a load, a clocked inverter, and a thirdtransistor, wherein a material of the first channel formation region isdifferent from that of the second channel formation region, wherein thethird transistor comprises a third gate electrode, a third sourceelectrode, a third drain electrode, and a third channel formationregion, wherein an input terminal of the clocked inverter iselectrically connected to the load and one of the first source electrodeand the first drain electrode through the first wiring, wherein anoutput terminal of the clocked inverter is connected to one of the thirdsource electrode and the third drain electrode, and wherein the other ofthe third source electrode and the third drain electrode is electricallyconnected to a power supply potential, the driving method comprising thesteps of: gradually changing a first potential input to the secondwiring from a second potential to a third potential; gradually changinga fourth potential input to the first wiring in accordance with thechange of the first potential, the fourth potential being determined bya resistance division of the load and the first transistor; inputting aclock signal and an inversed clock signal to the clocked inverterwhenever the first potential is changed; outputting an output signalfrom the clocked inverter whenever the clock signal and the inversedclock signal are input to the clocked inverter; and inputting the clocksignal or the inversed clock signal to the third gate electrode andturning off the third transistor while the clock signal or the inversedclock signal is input to the third gate electrode.
 12. The drivingmethod according to claim 11, wherein the first transistor is ap-channel transistor, and wherein the second transistor is an n-channeltransistor.
 13. The driving method according to claim 11, wherein thesecond channel formation region includes an oxide semiconductor.
 14. Thedriving method according to claim 11, wherein the semiconductor devicefurther comprises a latch circuit electrically connected to the outputterminal of the clocked inverter.
 15. The driving method according toclaim 11, wherein the first wiring is electrically connected to one ofthe second source electrode and the second drain electrode.
 16. Thedriving method according to claim 11, wherein the third transistor is ap-channel transistor, wherein the power supply potential is VDD, andwherein the inversed clock signal is input to the third gate electrodewhenever the first potential is changed.
 17. The driving methodaccording to claim 11, wherein the third transistor is an n-channeltransistor, wherein the power supply potential is GND, and wherein theclock signal is input to the third gate electrode whenever the firstpotential is changed.